drm/i915/display: Add HAS_MBUS_JOINING

This will make easy to extend MBUS joining support to future platforms
that also supports this feature.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331184152.1086943-1-jose.souza@intel.com