drm/vboxvideo: Use devm_gen_pool_create

Aside from deleting all the cleanup code we're now also setting a name
for the pool

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-7-daniel.vetter@ffwll.ch