i965: Simplify viewport extents programming on GEN8

Viewport extents are a 3rd rectangle that defines which pixels get
discarded as part of the rasterization process. The actual pixels drawn
to the screen are an intersection of the drawing rectangle, the viewport
extents, and the scissor rectangle. It permits the use of guardband
clipping in all cases (see later patch). The actual pixels drawn to the
screen are an intersection of the drawing rectangle, the viewport
extents, and the scissor rectangle.

Scissor rectangle is not super important for this discussion as it should
always help do the right thing provided the programmer uses it.

switch (viewport dimensions, drawrect dimension) {
   case viewport > drawing rectangle: no effects; break;
   case viewport == drawing rectangle: no effects; break;
   case viewport < drawing rectangle:
      Pixels (after the viewport transformation but before expensive
      rastersizing and shading operations) which are outside of the
      viewport are discarded.
}

I am unable to find a test case where this improves performance, but in
all my testing it doesn't hurt performance, and intuitively, it should
not ever hurt performance. It also permits us to use the guardband more
freely (see upcoming patch).

v2: Updating commit message.

v3: Commit message updates requested by Ken

Reviewed-by: Kenneth Graunke <[email protected]>

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