We have the same "override eDP VBT bpp with the current bpp" code
duplciated in two places. Extract it to a helper function.

TODO: Having this in .get_config() is pretty ugly. Should probably
try to move it somewhere else (setup_hw_state()/etc.)...
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510104242.6099-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>