drm/radeon: fix radeon_move_blit on 32bit systems

[ Upstream commit 13f479b9df4e2bbf2d16e7e1b02f3f55f70e2455 ]

This bug seems to be present for a very long time.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>