preserve CRTC{,2}_OFFSET_CNTL in 2D driver to avoid bad effects when

    pageflipping after a mode switch
take current page into account in AdjustFrame(); writing the CRTC offset
    via the CP was probably a bad idea as this can happen asynchronously,
    reverted
take frame offset into account when flipping pages
handle CRTC2 as well for pageflipping (untested)
preserve GEN_INT_CNTL on mode switches to prevent interrupts from getting
    disabled