drm/i915: Extract mbus_ctl_join_update()

drm/i915: Extract mbus_ctl_join_update()

We'll be wanting to reprogram the MBUS_CTL register during an
upcoming MBUS sanitation stage. Extract the reprogramming
into a helper that doesn't depend on the full atomic state
so that it can be reused.

We'll stick to sturct drm_i915_private for now (instead of
struct intel_display) since the rest of the skl wm code does the
same, and we might need to bakport this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241031155646.15165-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>