drm: fix MMU dependencies

DRM_VM and DRM_LEGACY shouldn't be selected if MMU isn't set.

Fixes: 62a0d98a ("drm: allow to use mmuless SoC")
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1484211456-5759-1-git-send-email-benjamin.gaignard@linaro.org