drm: Enable drm drivers to add drm sysfs devices.

Export utility functions for drivers to add specialized devices in the
sysfs drm class subdirectory.

Initially this will be needed form TTM to add a virtual device that
handles power management.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>