drm/i915/dpio: Introdude bxt_ddi_phy_rmw_grp()

Add a helper to do the "read from one per-lane register
and write to the group register" rmw cycle.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240412175818.29217-5-ville.syrjala@linux.intel.com


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