drm/radeon/cayman: add some missing regs to the VM reg checker

These regs were being wronly rejected leading to rendering
issues.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=56876



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>