drm/amdgpu_dm/crc: Implement verify_crc_source callback

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

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713135942.25061-5-mahesh1.kumar@intel.com