drm/amd/display: Drop atomic_obj_lock for private obj

[Why]
New DRM versions manage locking for private objects for us, so this
is no longer needed.

This also prevents a WARN_ON from occurring when the private object is
duplicated during the forced atomic commit that occurs from the HPD
handler.

The HPD handler calls drm_modeset_lock_all before the forced commit
and if the private object is duplicated then the
DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire) warning will be triggered
since we're trying to lock something when everything should have
already been locked.

[How]
Drop the lock and let DRM manage this.

Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1568,15 +1568,10 @@ static int dm_atomic_get_state(struct drm_atomic_state *state,
 	struct amdgpu_device *adev = dev->dev_private;
 	struct amdgpu_display_manager *dm = &adev->dm;
 	struct drm_private_state *priv_state;
-	int ret;
 
 	if (*dm_state)
 		return 0;
 
-	ret = drm_modeset_lock(&dm->atomic_obj_lock, state->acquire_ctx);
-	if (ret)
-		return ret;
-
 	priv_state = drm_atomic_get_private_obj_state(state, &dm->atomic_obj);
 	if (IS_ERR(priv_state))
 		return PTR_ERR(priv_state);
@@ -1683,8 +1678,6 @@ static int amdgpu_dm_mode_config_init(struct amdgpu_device *adev)
 
 	adev->ddev->mode_config.fb_base = adev->gmc.aper_base;
 
-	drm_modeset_lock_init(&adev->dm.atomic_obj_lock);
-
 	state = kzalloc(sizeof(*state), GFP_KERNEL);
 	if (!state)
 		return -ENOMEM;

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

@@ -132,8 +132,6 @@ struct amdgpu_display_manager {
 	 */
 	struct drm_private_obj atomic_obj;
 
-	struct drm_modeset_lock atomic_obj_lock;
-
 	/**
 	 * @dc_lock:
 	 *