exynos: create write-combining memory maps

We were always requesting uncacheable memory here. Oops.
Now we ask for write-combining, resulting in a return to the land of
normal 2D graphics speeds.

Need to check that the write-combine buffer gets cleared at appropriate
times.

Basic performance differences:

                |  BEFORE   BEFORE  |  AFTER   AFTER
                |  RAW X    MUTTER  |  RAW X   MUTTER
----------------+-------------------+----------------
glxgears (fps)  |  149      141     |  148     140
es2gears (fps)  |  153      177     |  313     290
glmark-es2      |  34       35      |  91      68
gtkperf (time)  |  20.47    23.3    |  10.7    13.6