drm/amdgpu: fix warning of drm_mm_clean

Kernel doorbell BOs needs to be freed before ttm_fini.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4145


Fixes: 54c30d2a ("drm/amdgpu: create kernel doorbell pages")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: ZhenGuo Yin <zhenguo.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 39938a8e)
Cc: stable@vger.kernel.org