drm/i915/edp: clean up code and comments around eDP DPCD read

Some minor drive-by cleanups.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171026142932.17737-2-jani.nikula@intel.com