i965/fs: Assume shadow sampler swizzling is <X, X, X, 1>.

i965/fs: Assume shadow sampler swizzling is <X, X, X, 1>.

Our previous assumption, SWIZZLE_XYZW, was completely bogus for depth
textures.  There are no Y, Z, or W components.

DEPTH_TEXTURE_MODE has three options:
- GL_LUMINANCE: <X, X, X, 1>
- GL_INTENSITY: <X, X, X, X>
- GL_ALPHA:     <0, 0, 0, X>

The default value is GL_LUMINANCE, and most applications don't seem to
alter DEPTH_TEXTURE_MODE.  Make that our precompile guess.

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