drm (ast, cirrus, mgag200, nouveau, savage, vmwgfx): Remove drm_mtrr_{add, del}

This replaces drm_mtrr_{add,del} with arch_phys_wc_{add,del}.  The
interface is simplified (because the base and size parameters to
drm_mtrr_del never did anything), and it no longer adds MTRRs on
systems that don't need them.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>