i965/vec4: Drop writemasks on scratch reads.

i965/vec4: Drop writemasks on scratch reads.

This code is complete nonsense and has apparently existed since I first
implemented register spilling in the VS two years ago.

Scratch reads are SEND messages, which ignore the destination writemask.

The comment about "data that may not have been written to scratch" is
also confusing - we always spill whole 4x2 registers, so such data
simply does not exist.  We can safely ignore the writemask.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>