i965: Enable fast clears on non-8x4-aligned sizes.

i965: Enable fast clears on non-8x4-aligned sizes.

Improves glb2.7 performance at a misaligned size by 2.3% +/- 0.7% (n=11).
The workaround was to avoid bad primitive/surface sizes, but that's worked
around as of a14dc4f92cdad6177d83f051a088a66e31a973bc.  (One might note
that pre-gen7 we don't know that the right half of an 8x4 at the right
edge is actually our pixels, but we're already clobbering those pixels for
depth resolves anyway and more work would be required to avoid that).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>