[radeonhd] [PATCH] Improve wait_vline range

Hi alex, the attached patch is an improvement to the wait_vline_range function. It adds a checking that the desired crtc is active and limits the given vline range to the visible are of the crtc, fxing the issue that the xserver gets a runaway process. Could you push this to the r6xx-r7xx-support branch? Thanks, Christian.