drm/i915: Extract intel_dbuf_mbus_join_update()

Extact the stuff that writes the joining bits in MBUS_CTL
into its own function. Will help with correctly sequencing
the operations done during mbus programming.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402155016.13733-8-ville.syrjala@linux.intel.com