xf86drm: fallback to MODALIAS for OF less platform devices

Some devices can lack OF data or it may not be available in the uevent
file. Fallback to the MODALIAS data in those cases.

We strip any leading "MODALIAS=.*:" thus the resulting information is
compatible with existing code in Mesa.

v2: foo -> tmp_name

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Lucas Stach <l.stach@pengutronix.de> (v1)