drm/vgem: Use drmm_add_final_kfree

With this we can drop the final kfree from the release function.

v2: After drm_dev_init/drmm_add_final_kfree we need to clean up
everything through a drm_dev_put. Rework the unwind code to match
that.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@chromium.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-14-daniel.vetter@ffwll.ch