radeon/pm: Guard access to rdev->pm.power_state array

It's never allocated on systems without an ATOMBIOS or COMBIOS ROM.

Should fix an oops I encountered while resetting the GPU after a lockup
on my PowerBook with an RV350.

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