drm/vc4: hdmi: Constify drm_display_mode

We don't modify the drm_display_mode pointer we have in the driver in
most places, so let's make them const.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220829134731.213478-2-maxime@cerno.tech