drm: Consistently return -ENOENT when a mode object can't be found

We tend to return -EINVAL for everything. Let's try to help poor
userland developers a bit by at least returning -ENONET for missing
objects.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>