drm: compile drm_vm.c only when needed

drm_vm.c functions are only need for DRM_LEGACY and DRM_NOUVEAU.
Use a new DRM_VM to define when drm_vm.c in needed.

stub drm_legacy_vma_flush() to avoid compilation issues

version 4:
- a "config DRM_VM" in Kconfig

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
[danvet: Fix conflict.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>