mrst: get video subdevices using a function

mrst: get video subdevices using a function

BZ: 16972

Different platforms supported by the mrst will have different
subdevices. Therefore the list of subdevices can not be obtained
from a fixed table.

Change-Id: I8a6f7fecf35a18f6840b93e4bb1b9591dd3a3ff9
Signed-off-by: Tuukka Toivonen <tuukka.toivonen@intel.com>
Reviewed-on: http://android.intel.com:8080/32611
Reviewed-by: Koski, Anttu <anttu.koski@intel.com>
Tested-by: Koski, Anttu <anttu.koski@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>