drm/vkms/crc: Implement verify_crc_source callback

This patch implements "verify_crc_source" callback function for
Virtual KMS drm driver.

Changes Since V1:
- update values_cnt in verify_crc_source
Changes Since V2:
- don't return early from set_crc_source to keep behavior same (Haneen)

Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180821083858.26275-2-mahesh1.kumar@intel.com