Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c: In function 'update_gpuvm_pte':
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c:840:20: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c:838:20: warning:
 variable 'vm' set but not used [-Wunused-but-set-variable]

They're never used since introduction in a46a2cd1 ("drm/amdgpu: Add GPUVM
memory management functions for KFD")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>