drm/amdgpu: cleanup gmc_v11_0_flush_gpu_tlb

Remove leftovers from copying this from the gmc v10 code.

v2: squash in fix from Yifan

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