drm: merge drm_drv.c into drm_ioctl.c

All that is left in drm_drv.c is ioctl management. Merge it into
drm_ioctl.c so we have all ioctl management in one file (and the name is
much more fitting).

Maybe we should now rename drm_stub.c to drm_drv.c again?

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>