Return an -ENODEV from drmGetDevice() when no device was found.

Fixes crashes in Mesa on platform devices, which expected *device to
have a device when 0 was returned.

(code from a paste by Rob, commit message by anholt)

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>