drm/amdgpu: fix and cleanup amdgpu_gem_object_close v4

drm/amdgpu: fix and cleanup amdgpu_gem_object_close v4

The problem is that we can't add the clear fence to the BO
when there is an exclusive fence on it since we can't
guarantee the the clear fence will complete after the
exclusive one.

To fix this refactor the function and also add the exclusive
fence as shared to the resv object.

v2: fix warning
v3: add excl fence as shared instead
v4: squash in fix for fence handling in amdgpu_gem_object_close

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: xinhui pan <xinhui.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>