kmssink: fix bad property memory management

When the driver-name property is read it incorrectly
transfers ownership of the underlying string to the
reader of the property, causing crashes.

This prevented using GST_DEBUG_DUMP_DOT_DIR to dump
pipeline graphs containing kmssink, as the dumping
process always reads all properties.

Change-Id: I5432ca1e3eca793153fdab93909a39bbc1698720
1 file changed