drm: move module initialization to drm_stub.c

Most of the new DRM management functions are nowadays in drm_stub.c. By
moving the core module initialization to drm_stub.c we can make several
global variables static and keep the stub-open helper local.

The core files now look like this:
  drm_stub.c: Core management
   drm_drv.c: Ioctl dispatcher
 drm_ioctl.c: Actual ioctl backends
  drm_fops.c: Char-dev file-operations

A follow-up patch will move what is left from drm_drv.c into drm_ioctl.c.

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