drm/amd/amdgpu: Remove division from vblank_wait

Removes division from vblank_wait for DCE v6, v8,
and v10.  Was already implemented for v11.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>