drm/amdgpu: drop the busy wait for GMC v9 TLB invalidations

This code is not performance critical.

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