drm/amd/display: call amdgpu_dm_fini when hw_fini.

to free up drm mode_config info.

fix issue: unload amdgpu, can't load amdgpu again.
[drm:drm_debugfs_init [drm]] *ERROR* Cannot create /sys/kernel/debug/dri/0
[drm:drm_minor_register [drm]] *ERROR* DRM: Failed to initialize /sys/kernel/debug/dri.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Andrey <Grodzovsky&lt;andrey.grodzovsky@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>