EXA support

Hello,

I've been adding EXA support to the xf86-video-sunffb and
xf86-video-suncg6 drivers, mostly because the next Xorg update is going
to drop XAA support. Status so far:
- sunffb works for basic acceleration but lacks xrender support, so it
  still defaults to XAA. Enable EXA with Option "AccelMethod" "exa".
- suncg6 works, basic acceleration and image uploads use the blitter.
  Some operations ( x11perf -comppixwin500 for example ) are MUCH
  faster that way, monochrome text drawing is slower ( EXA doesn't even
  try to accelerate that ). Off-screen memory usage is suboptimal since
  EXA expects blitters with variable pitch, which cg6 doesn't have.
  Defaults to EXA, still supports Option "AccelMethod" "xaa"

EXA support in both drivers is fairly new, so please test.

As for other drivers relevant here:
- suntcx and suncg14 are EXA only, never had XAA
- all ATI variants already have EXA support
- glint ( think PGX32, Raptor GFX etc. ) - no EXA support, fairly easy
  to add, will do when I get there unless upstream beats me to it
  ( which I doubt since Permedia2 is rather old and irrelevant on x86 )
- pnozz and ag10e are also XAA only so far but should be sufficiently
  obscure not to stop the update. EXA support for those is still fairly
  easy to add though. Will do when I get there.

have fun
Michael