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>
diff --git a/drivers/gpu/drm/drm_legacy.h b/drivers/gpu/drm/drm_legacy.h
index c6f422e..e4bb5ad 100644
--- a/drivers/gpu/drm/drm_legacy.h
+++ b/drivers/gpu/drm/drm_legacy.h
@@ -74,7 +74,14 @@ int drm_legacy_freebufs(struct drm_device *d, void *v, struct drm_file *f);
 int drm_legacy_mapbufs(struct drm_device *d, void *v, struct drm_file *f);
 int drm_legacy_dma_ioctl(struct drm_device *d, void *v, struct drm_file *f);
 
+#ifdef CONFIG_DRM_VM
 void drm_legacy_vma_flush(struct drm_device *d);
+#else
+static inline void drm_legacy_vma_flush(struct drm_device *d)
+{
+	/* do nothing */
+}
+#endif
 
 /*
  * AGP Support