drm/dev: Remove drm_dev_init

We can now also delete drm_dev_init, now that vkms, vgem and i915
selftests are resolved.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918132505.2316382-5-daniel.vetter@ffwll.ch