drm: Fix broken use of _PAGE_NO_CACHE on powerpc

That constant isn't meant to be used outside of arch mm code

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467018023.20278.65.camel@kernel.crashing.org