drm/i915: Add a FIXME about crtc !active vs. watermarks

When the crtc is enabled but !active, we should still compute the
watermarks as if the planes were visible. That would make it more
likely that the we can later transition to active without errors.

Add a FIXME to remind people that we're doing the wrong thing now.
We should perhaps just move the wm computation for each individual plane
into the .check_plane hook, and later we'd just combine the results from
all active planes.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461940278-17122-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -11823,6 +11823,11 @@ int intel_plane_atomic_calc_changes(struct drm_crtc_state *crtc_state,
 	 * Visibility is calculated as if the crtc was on, but
 	 * after scaler setup everything depends on it being off
 	 * when the crtc isn't active.
+	 *
+	 * FIXME this is wrong for watermarks. Watermarks should also
+	 * be computed as if the pipe would be active. Perhaps move
+	 * per-plane wm computation to the .check_plane() hook, and
+	 * only combine the results from all planes in the current place?
 	 */
 	if (!is_crtc_enabled)
 		to_intel_plane_state(plane_state)->visible = visible = false;