tests: add support for imx-drm

Since imx-drm has graduated from staging it seems to be a good idea to
recognize it by default in the libdrm tests.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>