drm/amdgpu: simplify the bo reference on amdgpu_bo_update

BO ptr already be initialized at definition, we needn't use the complicated
reference.

v2: fix typo at subject line

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>