drm/vboxvideo: Use managed interfaces for framebuffer write combining

Replace arch_phys_wc_add() with the rsp managed function. Allows for
removing the cleanup code for memory management

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210916181601.9146-6-tzimmermann@suse.de