i965/vec4: Override destination register writemask in sampler message send.

i965/vec4: Override destination register writemask in sampler message send.

This line was removed by accident in commit
16b911257440afbd77a6eb762e28df62e3c19bc7 causing a regression in the
ES3-CTS.gtf.GL3Tests.shadow.shadow_execution_vert Khronos conformance
test.  It's necessary because the swizzle_result() code below expects
all four components of the vector to be valid.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89094
Tested-by: Lu Hua <huax.lu@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>