drm/radeon: fix r1xx/r2xx register checker for POT textures

Shift and mask were reversed.  Noticed by chance.

Tested-by: Meelis Roos <mroos@linux.ee>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org