drm/i915/dp: split out aux functionality to intel_dp_aux.c

Split out the DP aux functionality to a new intel_dp_aux.[ch]. This is a
surprisingly clean cut.

v2:
- Remove intel_dp_pack_aux declaration from intel_dp.h (Anshuman)
- Fixed some whitespace/comment checkpatch warnings

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120101834.19813-4-jani.nikula@intel.com