drm/i915/dp: use readb and writeb calls for single byte DPCD access

This is what we have the readb and writeb variants for. Do some minor
return value and variable cleanup while at it.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: 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: http://patchwork.freedesktop.org/patch/msgid/fd8a8f110bcfdc73a8c9241e5f9d61f7dd7c9677.1491485983.git.jani.nikula@intel.com