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

commit 008037d4d972c9c47b273e40e52ae34f9d9e33e7 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>