svga: Prefer accelerated blits over cpu copy region

svga: Prefer accelerated blits over cpu copy region

This reduces the number of cpu copy_region fallbacks on a Nvidia system
running the piglit command

./publish/bin/piglit run  -1 -t copy -t blit tests/quick

from 64789 to 780

Previously this has caused a regression in piglit test
spec@!opengl 1.0@gl-1.0-scissor-copypixels, but I'm currently not able to
reproduce that regression.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>