i965: Support MESA_FORMAT_SIGNED_RGBA_16.

i965: Support MESA_FORMAT_SIGNED_RGBA_16.

The hardware supports this format with no known quirks, so we may as
well enable it.

Alpha blending is not supported until Sandybridge, but as far as I can
tell, OpenGL doesn't require alpha blending on SNORM formats.  Plus, we
already expose R8G8B8A8_SNORM which has a similar restriction.

Fixes 6 piglit texwrap-2D-*SNORM* cases,
gl-3.1/required-sized-texture-formats, and 10 oglconform snorm-textures
subcases

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>