kms causes broken vsync with r600.

[rdxijn at yandex.ru]

Enabling kms with r600 causes serious problems with vsync. For example, glxgears with low-res window works just as there is no vsync, and with large window fps becomes 60, but visible and regular lags appear - rendering just stop approximately once a second for a sort moment.  EXAVSync options seems has no effect. Also there is following error when starting gl apps:

do_wait: drmWaitVBlank returned -1, IRQs don't seem to be working correctly.
Try adjusting the vblank_mode configuration parameter.

Adjusting vblank_mode var doesn't work ether. With ums there is no sync problem.

I'm using Debian Squeeze with mesa 7.7.1-1 and radeon 6.13.0-1. Also i have two 1920x1080 panels connected via dvi.