drm: add of_graph endpoint helper to find possible CRTCs

Add a helper to allow encoders to find their possible CRTCs from the
OF graph without having to re-implement this functionality.  We add a
device_node to drm_crtc which corresponds with the port node in the
DT description of the CRTC device.

We can then scan the DRM device list for CRTCs to find their index,
matching the appropriate CRTC using the port device_node, thus building
up the possible CRTC mask.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>