drm/amd/display: Add NULL checks for vblank workqueue

[Why]
If we're running a headless config with 0 links then the vblank
workqueue will be NULL - causing a NULL pointer exception during
any commit.

[How]
Guard access to the workqueue if it's NULL and don't queue or flush
work if it is.
Reported-by: Mike Lothian <mike@fireburn.co.uk>
BugLink: https://gitlab.freedesktop.org/drm/amd/-/issues/1700
Fixes: 58aa1c50 ("drm/amd/display: Use vblank control events for PSR enable/disable")
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -6185,6 +6185,7 @@ static inline int dm_set_vblank(struct drm_crtc *crtc, bool enable)
 		return 0;
 
 #if defined(CONFIG_DRM_AMD_DC_DCN)
+	if (dm->vblank_control_workqueue) {
 		work = kzalloc(sizeof(*work), GFP_ATOMIC);
 		if (!work)
 			return -ENOMEM;
@@ -6200,6 +6201,7 @@ static inline int dm_set_vblank(struct drm_crtc *crtc, bool enable)
 		}
 
 		queue_work(dm->vblank_control_workqueue, &work->work);
+	}
 #endif
 
 	return 0;
@@ -8809,6 +8811,7 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 		 * If PSR or idle optimizations are enabled then flush out
 		 * any pending work before hardware programming.
 		 */
+		if (dm->vblank_control_workqueue)
 			flush_workqueue(dm->vblank_control_workqueue);
 #endif
 
@@ -9144,6 +9147,7 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
 		/* if there mode set or reset, disable eDP PSR */
 		if (mode_set_reset_required) {
 #if defined(CONFIG_DRM_AMD_DC_DCN)
+			if (dm->vblank_control_workqueue)
 				flush_workqueue(dm->vblank_control_workqueue);
 #endif
 			amdgpu_dm_psr_disable_all(dm);