drm/vmwgfx: Stop using plane->fb in vmw_kms_update_implicit_fb()

The only caller of vmw_kms_update_implicit_fb() is the page_flip
hook which itself gets called with the plane mutex already held.
Hence we can look at plane->state safely.

v2: Drop the bogus lockdep assert

Cc: Deepak Rawat <drawat@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-4-ville.syrjala@linux.intel.com
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -2803,6 +2803,7 @@ void vmw_kms_update_implicit_fb(struct vmw_private *dev_priv,
 				struct drm_crtc *crtc)
 {
 	struct vmw_display_unit *du = vmw_crtc_to_du(crtc);
+	struct drm_plane *plane = crtc->primary;
 	struct vmw_framebuffer *vfb;
 
 	mutex_lock(&dev_priv->global_kms_state_mutex);
@@ -2810,7 +2811,7 @@ void vmw_kms_update_implicit_fb(struct vmw_private *dev_priv,
 	if (!du->is_implicit)
 		goto out_unlock;
 
-	vfb = vmw_framebuffer_to_vfb(crtc->primary->fb);
+	vfb = vmw_framebuffer_to_vfb(plane->state->fb);
 	WARN_ON_ONCE(dev_priv->num_implicit != 1 &&
 		     dev_priv->implicit_fb != vfb);