Remove throttling from radeon_dri2_copy_region2

Throttling should be handled by the client-side drivers.

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
(Ported from amdgpu commit 8a34a8149860ac15e83ccdbd8d9a527d8d3e5997)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>