drm/i915: Relocate intel_mbus_dbox_update()

intel_mbus_dbox_update() will become static soon. Relocate it
into a place that avoids having to add a forward declaration
for it.

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/20240402155016.13733-7-ville.syrjala@linux.intel.com