drm: enable render-nodes by default

We introduced render-nodes about 1/2 year ago and no problems showed up.
Remove the drm_rnodes argument and enable them by default now.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>