drm/amdgpu: introduce runtime pm mode

It can benefit code consistency in future.

Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>