drisw: Simplify GC setup

drisw: Simplify GC setup

There's no reason to have two GCs here. The only difference between
them is that swapgc would generate graphics exposures, except we only
ever use this GC for PutImage, and PutImage doesn't generate graphics
exposures. We also don't need to explicitly ChangeGC to GXCopy, because
that's the default.

Reviewed-by: Eric Anholt <eric@anholt.net>