drm/amdgpu: Delete an unnecessary check before drm_gem_object_unreference_unlocked()

The drm_gem_object_unreference_unlocked() function tests whether
its argument is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -516,9 +516,7 @@ static void amdgpu_user_framebuffer_destroy(struct drm_framebuffer *fb)
 {
 	struct amdgpu_framebuffer *amdgpu_fb = to_amdgpu_framebuffer(fb);
 
-	if (amdgpu_fb->obj) {
-		drm_gem_object_unreference_unlocked(amdgpu_fb->obj);
-	}
+	drm_gem_object_unreference_unlocked(amdgpu_fb->obj);
 	drm_framebuffer_cleanup(fb);
 	kfree(amdgpu_fb);
 }