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

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>