xf86drm: add drmGetDeviceFromDevId

This adds a function to get a drmDevicePtr from a dev_t identifier
of a device. This is useful for Wayland that uses these to identify
devices over the protocol.

This is done by taking the implementation of drmGetDevice2, and removing
the call to fstat to find the dev_t.

Signed-off-by: Scott Anderson <scott.anderson@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniels@collabora.com>