drm/i915/dp: return number of bytes written for short aux/i2c writes
Allow for a larger receive data size, and check if the receiver returned
the number of bytes written. Without this, we've basically skipped all
the unwritten bytes for short writes.
Signed-off-by: Jani Nikula <jani.nikula@intel.com> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>