drm/radeon: add a check to wait_reg_mem command

WAIT_REG_MEM on register does not allow the use of PFP.
Enforce this restriction when checking packets sent from
userland.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>