drm/amdgpu: add amdgpu_gfx_state_change_set() set gfx power change entry (v2)

The new amdgpu_gfx_state_change_set() funtion can support set GFX power
change status to D0/D3.

v2: squash in warning fix (Alex)

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>