i965: Make the precompile ignore DEPTH_TEXTURE_MODE on Gen7.5+.

i965: Make the precompile ignore DEPTH_TEXTURE_MODE on Gen7.5+.

Gen7.5+ platforms that support the "Shader Channel Select" feature leave
key->tex.swizzles[i] as SWIZZLE_NOOP except when GL_DEPTH_TEXTURE_MODE
is GL_ALPHA (which is really uncommon).  So, the precompile should leave
them as SWIZZLE_NOOP (aka SWIZZLE_XYZW) as well.

We didn't notice this because prog->ShadowSamplers is not set correctly.
The next patch will fix that problem.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87886
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>