drm/vkms: add crc sources list

Other drivers are able to list crc sources when accessing
/sys/kernel/debug/dri/.../crtc-0/crc/control

Even though VKMS now supports only 'auto' mode, it is more consistent to
have the list available to the userspace.

Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613121802.2193-3-oleg.vasilev@intel.com