drm/amdgpu: fix the nullptr issue as for PWR IP not existing in discovery table

Fixes: c1cf79ca ("drm/amdgpu: use IP discovery table for renoir")

This nullptr issue should be specific on the Renoir series during try access the PWR_MISC_CNTL_STATUS
when PWR IP not been detected by discovery table. Moreover the PWR IP not existing in Renoir series is
expected therefore just avoid access PWR register in Renoir series.

Signed-off-by: Prike.Liang <Prike.Liang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>