drm/amdgpu: always flush the TLB on gfx8

The TLB on GFX8 stores each block of 8 PTEs where any of the valid bits
are set.

Fixes: 5255e146 ("drm/amdgpu: rework TLB flushing")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>