drm/amdgpu/dc: Stop updating plane->fb

We want to get rid of plane->fb on atomic drivers. Stop setting it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-8-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

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

@@ -3951,8 +3951,6 @@ static void amdgpu_dm_do_flip(struct drm_crtc *crtc,
 
 	/* Flip */
 	spin_lock_irqsave(&crtc->dev->event_lock, flags);
-	/* update crtc fb */
-	crtc->primary->fb = fb;
 
 	WARN_ON(acrtc->pflip_status != AMDGPU_FLIP_NONE);
 	WARN_ON(!acrtc_state->stream);