drm: gc now dead mode_group code

Two nice things here:
- drm_dev_register will truly register everything in the right order
  if the driver doesn't have a ->load callback. Before this we had to
  init the primary mode_group after the device nodes where already
  registered.

- Less things to keep track of when reworking the connector locking,
  yay!

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>