drm: rcar-du: Split CRTC handling to support hardware indexing

The DU CRTC driver does not support distinguishing between a hardware
index, and a software (CRTC) index in the event that a DU channel might
not be populated by the hardware.

Support this by adapting the rcar_du_device_info structure to store a
bitmask of available channels rather than a count of CRTCs. The count
can then be obtained by determining the hamming weight of the bitmask.

This allows the rcar_du_crtc_create() function to distinguish between
both index types, and non-populated DU channels will be skipped without
leaving a gap in the software CRTC indexes.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>