drm/amdgpu/pm: fix the Stable pstate Test in amdgpu_test

If GFX DPM is disbaled, Stable pstate Test in amdgpu_test fails.
Check GFX DPM statue before change clock level

Log:
[   46.595274] [drm] Initialized amdgpu 3.46.0 20150101 for 0000:02:00.0 on minor 0
[   46.599929] fbcon: amdgpudrmfb (fb0) is primary device
[   46.785753] Console: switching to colour frame buffer device 240x67
[   46.811765] amdgpu 0000:02:00.0: [drm] fb0: amdgpudrmfb frame buffer device
[  131.398407] amdgpu 0000:02:00.0: amdgpu: Failed to set performance level!

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>