Revert/modify some RandR changes to reenable XAA.
Revert/modify some RandR changes to reenable XAA.

  XAA was not initialized anymore in RandR patch, but it is still the
default accel method. Also reenable offscreen fb manager.
  At least on the MSOC OEM sample (using X Server 1.4), needs some more
work as EXA is crashing in a call to exaPrepareAccess() when switching
to a terminal; this seems to be related to disabling exa offscreen
pixmaps on the smi501.

lumag/xf86-video-siliconmotion,lumag/xf86-video-siliconmotion