drm/amd/display: fix issues with driver unload

Currently, we run into a number of WARN()s when attempting to unload the
amdgpu driver (e.g. using "modprobe -r amdgpu"). These all stem from
calling drm_encoder_cleanup() too early. So, to fix this we can stop
calling drm_encoder_cleanup() from amdgpu_dm_fini() and instead have it
be called from amdgpu_dm_encoder_destroy(). Also, we don't need to free
in amdgpu_dm_encoder_destroy() since mst_encoders[] isn't explicitly
allocated by the slab allocator.

Fixes: f74367e4 ("drm/amdgpu/display: create fake mst encoders ahead of time (v4)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1730,10 +1730,6 @@ static void amdgpu_dm_fini(struct amdgpu_device *adev)
 		adev->dm.vblank_control_workqueue = NULL;
 	}
 
-	for (i = 0; i < adev->dm.display_indexes_num; i++) {
-		drm_encoder_cleanup(&adev->dm.mst_encoders[i].base);
-	}
-
 	amdgpu_dm_destroy_drm_device(&adev->dm);
 
 #if defined(CONFIG_DRM_AMD_SECURE_DISPLAY)

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

@@ -468,7 +468,6 @@ static const struct drm_connector_helper_funcs dm_dp_mst_connector_helper_funcs
 static void amdgpu_dm_encoder_destroy(struct drm_encoder *encoder)
 {
 	drm_encoder_cleanup(encoder);
-	kfree(encoder);
 }
 
 static const struct drm_encoder_funcs amdgpu_dm_encoder_funcs = {