drm/amd/display: avoid using null object of framebuffer

Instead of using state->fb->obj[0] directly, get object from framebuffer
by calling drm_gem_fb_get_obj() and return error code when object is
null to avoid using null object of framebuffer.

Fixes: 5d945cbcd4b1 ("drm/amd/display: Create a file dedicated to planes")
Signed-off-by: Ma Ke <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_plane.c

···
 #include <drm/drm_blend.h>
 #include <drm/drm_gem_atomic_helper.h>
 #include <drm/drm_plane_helper.h>
+#include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_fourcc.h>
 
 #include "amdgpu.h"
···
 	}
 
 	afb = to_amdgpu_framebuffer(new_state->fb);
-	obj = new_state->fb->obj[0];
+	obj = drm_gem_fb_get_obj(new_state->fb, 0);
+	if (!obj) {
+		DRM_ERROR("Failed to get obj from framebuffer\n");
+		return -EINVAL;
+	}
+
 	rbo = gem_to_amdgpu_bo(obj);
 	adev = amdgpu_ttm_adev(rbo->tbo.bdev);
-
 	r = amdgpu_bo_reserve(rbo, true);
 	if (r) {
 		dev_err(adev->dev, "fail to reserve bo (%d)\n", r);