drm: Garbage collect drm_dev_fini

It has become empty. Given the few users I figured not much point
splitting this up.

v2: Rebase over i915 changes.

v3: Rebase over patch split fix.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-26-daniel.vetter@ffwll.ch