drm/rcar-du/crc: Implement get_crc_sources callback

This patch implements get_crc_sources callback, which returns list of
all the crc sources supported by driver in current platform.

Changes Since V1:
 - move sources list per-crtc
 - init sources-list only for gen3
Changes Since V2:
 - Adopt to driver style
 - Address other review comments from Laurent Pinchart
Changes Since V3/4/5: (Laurent Pinchart review)
 - s/rcar_du_crtc_crc_sources_list_init/rcar_du_crtc_crc_init
 - s/rcar_du_crtc_crc_sources_list_uninit/rcar_du_crtc_crc_cleanup
 - other cleanup

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180808152630.6563-1-mahesh1.kumar@intel.com