drm: don't take modeset locks in getfb ioctl

We only need to push the fb unreference a bit down. While at it,
properly pass the return value from ->create_handle back to userspace.

Most drivers either return -ENODEV if they don't have a concept of
buffer objects (ast, cirrus, ...) or just install a handle for the
underlying gem object (which is ok since we hold a reference on that
through the framebuffer).

v2: Split out the ->create_handle rework in the individual drivers.
Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_crtc.c

@@ -2542,19 +2542,14 @@ int drm_mode_getfb(struct drm_device *dev,
 {
 	struct drm_mode_fb_cmd *r = data;
 	struct drm_framebuffer *fb;
-	int ret = 0;
+	int ret;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EINVAL;
 
-	drm_modeset_lock_all(dev);
 	fb = drm_framebuffer_lookup(dev, r->fb_id);
-	if (!fb) {
-		ret = -EINVAL;
-		goto out;
-	}
-	/* fb is protect by the mode_config lock, so drop the ref immediately */
-	drm_framebuffer_unreference(fb);
+	if (!fb)
+		return -EINVAL;
 
 	r->height = fb->height;
 	r->width = fb->width;
@@ -2566,8 +2561,8 @@ int drm_mode_getfb(struct drm_device *dev,
 	else
 		ret = -ENODEV;
 
-out:
-	drm_modeset_unlock_all(dev);
+	drm_framebuffer_unreference(fb);
+
 	return ret;
 }