drm/radeon: clean up atpx power control handling

The presence of the power control method should be determined
via the presence of the method in function 0.  However, some
sbioses only set the appropriate bits in function 1 so use
then to override a missing power control function.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>