drm/amdgpu: conduct a proper cleanup of PDB bo

drm/amdgpu: conduct a proper cleanup of PDB bo

BugLink: https://bugs.launchpad.net/bugs/1969857
[ Upstream commit 2d505453f38e18d42ba7d5428aaa17aaa7752c65 ]

Use amdgpu_bo_free_kernel instead of amdgpu_bo_unref to
perform a proper cleanup of PDB bo.

v2: update subject to be more accurate

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>