drm/mgag200: Use managed interfaces for framebuffer write combining

Replace arch_phys_wc_add() and arch_io_reserve_memtype_wc() with
the rsp managed functions. 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/20210916181601.9146-5-tzimmermann@suse.de