drm: add properties for suggested x/y offset for connectors. (v2)

Virtual GPUs would like to give the guest some indication where on the screen
the outputs are layed out. So far we only provide modes, these
properties could be exposed to userspace so the desktop environment
could use them as hints to set the correct offsets.

v2: rename properties to be more consistent.

Signed-off-by: Dave Airlie <airlied@redhat.com>