drm/i915: Extract pipe_mbus_dbox_ctl()

We'll be wanting to reprogram the PIPE_MBUS_DBOX_CTL registers
during an upcoming MBUS sanitation stage. To make that easier
extract a helper that computes the full register value for us.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241031155646.15165-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>