svga: fix texture swizzle writemasking

Commit bfe1e7737a76e3b046 changed how texture swizzles are set up.
This exposed a latent bug in the VMware driver: we were ignoring
the texture instruction's writemask when applying the 0 and 1
swizzle terms.

This wasn't caught by the Piglit texture swizzle test because it
only exercises fixed function (no write masking).

Fixes issues seen with ETQW apitrace.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
1 file changed