drm: Make the prime vmap/vunmap hooks optional.

Some drivers leave these unimplemented, so don't make them have
unimplemented stubs.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424004610.4637-2-eric@anholt.netReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_prime.c

@@ -409,7 +409,10 @@ void *drm_gem_dmabuf_vmap(struct dma_buf *dma_buf)
 	struct drm_gem_object *obj = dma_buf->priv;
 	struct drm_device *dev = obj->dev;
 
+	if (dev->driver->gem_prime_vmap)
 		return dev->driver->gem_prime_vmap(obj);
+	else
+		return NULL;
 }
 EXPORT_SYMBOL(drm_gem_dmabuf_vmap);
 
@@ -426,6 +429,7 @@ void drm_gem_dmabuf_vunmap(struct dma_buf *dma_buf, void *vaddr)
 	struct drm_gem_object *obj = dma_buf->priv;
 	struct drm_device *dev = obj->dev;
 
+	if (dev->driver->gem_prime_vunmap)
 		dev->driver->gem_prime_vunmap(obj, vaddr);
 }
 EXPORT_SYMBOL(drm_gem_dmabuf_vunmap);