i965: Unroll SIMD16 DDY_FINE on Sandybridge.

i965: Unroll SIMD16 DDY_FINE on Sandybridge.

This fixes 10 dEQP-GLES3 subtests:
dEQP-GLES3.functional.shaders.derivate.dfdy.texture.float_nicest.*.

Matt noticed that our Piglit tests for this use even numbered registers,
while the failing dEQP tests use odd numbered registers.  We believe
that it works for even numbered registers, but not otherwise.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>