drm/amdgpu: call flush_gpu_tlb directly in gfxhub enable

Here since we are in reset and takes the reset_domain write side lock
already. We can't use the flush tlb helper which tries to take the read
side.

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