When adding the max plane size checks to the .mode_valid() hooks
I naturally forgot about MST. Take care of that one as well.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Sean Paul <sean@poorly.run>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 2d20411e ("drm/i915: Don't advertise modes that exceed the max plane size")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191001154629.11063-1-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>