svga: fix swizzling for texture gather

svga: fix swizzling for texture gather

Texture swizzling for texture gather needs to be done to the selected texels
rather than to the returned vector. This patch has specical cases
for the different swizzles in emit_tg4().

Fixes a lot of piglit texture gather tests.

Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>