drm/i915/dp: make intel_dp_pack_aux() static again

The last user of intel_dp_pack_aux() outside intel_dp_aux.c got removed
in commit ad26451a ("drm/i915/display: Drop PSR support from HSW and
BDW"). Make the function static again.

Rename the pack/unpack functions to follow the usual naming conventions
while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112105703.1151391-1-jani.nikula@intel.com