MFC r254817:

drm: Import drm_dp_helper.c from Linux 3.8-rc3

While here, update drm_dp_helper.h to better match Linux one.
MFC r254818:

drm: Move definition of EREMOTEIO to drmP.h

It will be used by both i915 and radeon drivers.

Add ERESTARTSYS definition at the same time.
MFC r254819:

drm: Don't delete already deleted iicbus child from drm_iic_dp_aux

The iic_dp_aux_detach callback is therefore useless: it's replaced by
bus_generic_detach. This fixes a "General protection fault" panic during
second (incorrect) deletion of the child.

Tested by:	kwm@
Reviewed by:	ray@