drm/amdgpu: fix drm panic null pointer when driver not support atomic

When driver not support atomic, fb using plane->fb rather than
plane->state->fb.

Fixes: fe151ed ("drm/amdgpu: add generic display panic helper code")
Signed-off-by: Lu Yao <yaolu@kylinos.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 2f2a72d)

Author: Lu Yao

drivers/gpu/drm/amd/amdgpu/amdgpu_display.c

@@ -1880,7 +1880,12 @@ int amdgpu_display_get_scanout_buffer(struct drm_plane *plane,
 				      struct drm_scanout_buffer *sb)
 {
 	struct amdgpu_bo *abo;
-	struct drm_framebuffer *fb = plane->state->fb;
+	struct drm_framebuffer *fb;
+
+	if (drm_drv_uses_atomic_modeset(plane->dev))
+		fb = plane->state->fb;
+	else
+		fb = plane->fb;
 
 	if (!fb)
 		return -EINVAL;