i965: Switch blit color clears to tri clears on gen4/5.

i965: Switch blit color clears to tri clears on gen4/5.

Our understanding is that the 3D engine is supposed to be faster
anyway.  We used to have more overhead in our tri clear path than we
do today, which would have led to this choice.  But given that we
almost always see a depth clear along with a color clear, the path was
hardly exercised anyway.

Also, the color mask logic was broken in the presence of
GL_EXT_draw_buffers2's per-buffer colormask.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>