i965/vec4: Don't trim writemasks of texture instructions.

i965/vec4: Don't trim writemasks of texture instructions.

It was my understanding that the writemask works in SIMD4x2 mode for
texturing instructions and doesn't require a message header. Some bit of
this logic must be wrong, so disable it until it's understood.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76617
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>