drm/i915: Constify all plane_funcs structs

plane_funcs can be cosnt. Make them so.

v2: Rebase due to per-platforms plane_funcs

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-2-ville.syrjala@linux.intel.com