i965: Compute VS attribute WA bits earlier and check if they changed.

BRW_NEW_VERTICES is flagged every time we draw a primitive.  Having
the brw_vs_prog atom depend on BRW_NEW_VERTICES meant that we had to
compute the VS program key and do a program cache lookup for every
single primitive.  This is painfully expensive.

The workaround bit computation is almost entirely based on the vertex
attribute arrays (brw->vb.inputs[i]), which are set by brw_merge_inputs.
The only thing it uses the VS program for is to see which VS inputs are
actually read.  brw_merge_inputs() happens once per primitive, and can
safely look at the currently bound vertex program, as it doesn't change
in the middle of a draw.

This patch moves the workaround bit computation to brw_merge_inputs(),
right after assigning brw->vb.inputs[i], and stores the previous WA bit
values in the context.  If they've actually changed from the last draw
(which is uncommon), we signal that we need a new vertex program,
causing brw_vs_prog to compute a new key.

Improves performance in Gl32Batch7 by 13.6123% +/- 0.739652% (n=166)
on Haswell GT3e.  I'm told Baytrail shows similar gains.

v2: Introduce a new BRW_NEW_VS_ATTRIB_WORKAROUNDS dirty bit, rather
    than reusing BRW_NEW_VERTEX_PROGRAM (suggested by Chris Forbes).
    This prevents unnecessary re-emission of surface/sampler related
    atoms (and an SOL atom on Sandybridge).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chris Forbes <[email protected]>

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