drm/i915: Remove useless casts to intel_plane_state

The visible member used to be in intel_plane_state->visible,
but has been moved to drm_plane_state->visible. In the conversion
some casts were left in that are now useless.

to_intel_plane_state(x)->base.visible is the same as x->visible,
so use the latter to clear up the code a little.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484214225-30328-1-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_fbc.c

@@ -1296,7 +1296,7 @@ void intel_fbc_init_pipe_state(struct drm_i915_private *dev_priv)
 
 	for_each_intel_crtc(&dev_priv->drm, crtc)
 		if (intel_crtc_active(crtc) &&
-		    to_intel_plane_state(crtc->base.primary->state)->base.visible)
+		    crtc->base.primary->state->visible)
 			dev_priv->fbc.visible_pipes_mask |= (1 << crtc->pipe);
 }