drm/amdgpu/gfx10: remove unnecessary waiting on gfx inactive

The following KIQ ring test could guarantee the previous unmap
has been done.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>