tests/util: Add mxsfb-drm driver

Add an entry for the "mxsfb-drm" driver, so that the test utilities
work with the mxsfb driver without passing the -M argument.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>