Convert the BOCHS and CIRRUS drivers to atomic mode-setting

by Zhao Junwang for X.Org Foundation

Atomic mode-setting has been discussed and designed for literally years, now it's finally there, it is not only necessary but allows a bunch of nice and long overdue cleanup and unification. There are a lot ongoing atomic mode-setting conversions and some already committed to drm-next ones, while there still are some subsystems haven't started the work, as far as I know, radeon, udl, bochs and cirrus are some optional drivers. I will take DRM_BOCHS and DRM_CIRRUS_QEMU as my converting goals.