i965: Always scissor on Gen6-7.5 instead of disabling guardband.

i965: Always scissor on Gen6-7.5 instead of disabling guardband.

Previously we disabled the guardband when the viewport was smaller than
the framebuffer on Gen6-7.5, to prevent portions of primitives from
being draw outside of the viewport.  On Gen8+, we relied on the viewport
extents test to effectively scissor this away for us.

We can simply always enable scissoring instead.  We already include the
viewport in the scissor rectangle, so this will effectively do the
viewport extents test for us.  (The only difference is that the scissor
rectangle doesn't support sub-pixel values.  I think that's okay.)

Given that the viewport extents test is essentially a second scissor,
and is enabled for basically all 3D drawing on Gen8+, it stands to
reason that scissoring is cheap.  Enabling the guardband reduces the
cost of clipping, which is expensive.

The Windows driver appears to never disable guardband clipping, and
appears to use scissoring in this case.  I don't know if they leave
it on universally though.

This fixes misrendering in Blender, where the "floor plane" grid lines
started rendering at wrong angles after I disabled XY clipping of line
primitives.  Enabling the guardband seems to solve the issue.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99339
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>