svga: Fix a leftover debug hack

Fix what appears to be a leftover debug hack.
The hack would force the driver to take a different blit path; possibly,
although unverified, reverting to software blits.

Tested using piglit tests/quick. No related regressions.

Cc: "17.2 17.3 18.0" <mesa-stable@lists.freedesktop.org>
Fixes: 9d81ab7376 (svga: Relax the format checks for copy_region_vgpu10 somewhat)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104625
Reported-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>