drm/amd/display: drop vblank_lock from struct amdgpu_display_manager

As of commit 09a5df6c ("drm/amd/display: Fix multi-display support
for idle opt workqueue"), vblank_lock is no longer being used. So, don't
init it in amdgpu_dm_init() and remove it from struct
amdgpu_display_manager.
Reviewed-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NHamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -1399,7 +1399,6 @@ static int amdgpu_dm_init(struct amdgpu_device *adev)
 
 	mutex_init(&adev->dm.dc_lock);
 	mutex_init(&adev->dm.audio_lock);
-	spin_lock_init(&adev->dm.vblank_lock);
 
 	if(amdgpu_dm_irq_init(adev)) {
 		DRM_ERROR("amdgpu: failed to initialize DM IRQ support.\n");

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

@@ -365,13 +365,6 @@ struct amdgpu_display_manager {
 	 */
 	struct mutex audio_lock;
 
-	/**
-	 * @vblank_lock:
-	 *
-	 * Guards access to deferred vblank work state.
-	 */
-	spinlock_t vblank_lock;
-
 	/**
 	 * @audio_component:
 	 *