i965: Fix start/base_vertex_location for >1 prims but !BRW_NEW_VERTICES.

This is a partial revert of c89306983c07e5a88c0d636267e5ccf263cb4213.
It split the {start,base}_vertex_location handling into several steps:

1. Set brw->draw.start_vertex_location = prim[i].start
   and brw->draw.base_vertex_location = prim[i].basevertex.
   (This happened once per _mesa_prim, in the main drawing loop.)
2. Add brw->vb.start_vertex_bias and brw->ib.start_vertex_offset
   appropriately.  (This happened in brw_prepare_shader_draw_parameters,
   which was called just after brw_prepare_vertices, as part of state
   upload, and only happened when BRW_NEW_VERTICES was flagged.)
3. Use those values when emitting 3DPRIMITIVE (once per _mesa_prim).

If we drew multiple _mesa_prims, but didn't flag BRW_NEW_VERTICES on
the second (or later) primitives, we would do step #1, but not #2.
The first _mesa_prim would get correct values, but subsequent ones
would only get the first half of the summation.

The reason I originally did this was because I needed the value of
gl_BaseVertexARB to exist in a buffer object prior to uploading
3DSTATE_VERTEX_BUFFERS.  I believed I wanted to upload the value
of 3DPRIMITIVE's "Base Vertex Location" field, which was computed
as: (prims[i].indexed ? prims[i].start : prims[i].basevertex) +
brw->vb.start_vertex_bias.  The latter value wasn't available until
after brw_prepare_vertices, and the former weren't available in the
state upload code at all.  Hence the awkward split.

However, I believe that including brw->vb.start_vertex_bias was a
mistake.  It's an extra bias we apply when uploading vertex data into
VBOs, to move [min_index, max_index] to [0, max_index - min_index].

>From the GL_ARB_shader_draw_parameters specification:
"<gl_BaseVertexARB> holds the integer value passed to the <baseVertex>
 parameter to the command that resulted in the current shader
 invocation.  In the case where the command has no <baseVertex>
 parameter, the value of <gl_BaseVertexARB> is zero."

I conclude that gl_BaseVertexARB should only include the baseVertex
parameter from glDraw*Elements*, not any internal biases we add for
optimization purposes.

With that in mind, gl_BaseVertexARB only needs prim[i].start or
prim[i].basevertex.  We can simply store that, and go back to computing
start_vertex_location and base_vertex_location in brw_emit_prim(), like
we used to.  This is much simpler, and should actually fix two bugs.

Fixes missing geometry in Unvanquished.

Cc: "10.4 10.3" <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85529
Signed-off-by: Kenneth Graunke <[email protected]>
Acked-by: Ian Romanick <[email protected]>
Reviewed-by: Chris Forbes <[email protected]>

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