Reduce this maintenance nightmare a bit by converting the plane
min/max width/height stuff into vfuncs.

Now, if I could just think of a nice way to also use this for
intel_mode_valid_max_plane_size()...
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924185113.30849-1-ville.syrjala@linux.intel.comReviewed-by: Aditya Swarup <aditya.swarup@intel.com>