drm/rcar-du/crc: Implement verify_crc_source callback

This patch implements "verify_crc_source" callback function for
rcar drm driver.

Changes Since V1:
 - avoid duplication of code
Changes Since V2:
 - further optimize the code
Changes Since V3:
 - Adopt to driver style
 - Address review comments from Laurent Pinchart

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.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/20180723103824.21734-1-mahesh1.kumar@intel.com