drm/i915/ddi: Optimize mtl_port_buf_ctl_program

A small optimization and cleanup for mtl_port_buf_ctl_program function
which lets use intel_de_rmw instead of a intel_de_read and
intel_de_write.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250103051705.145161-3-suraj.kandpal@intel.com