Only back pixmaps requested for use with DRI2

The armsoc driver was backing all pixmaps with DRM allocations,
but as many of these pixmaps will never be accelerated by anything,
this forms quite a heavy performance overhead.
http://ssvb.github.io/2013/02/01/new-xf86-video-sunxifb-ddx-driver.html

Adopt the fbturbo model where we only DRM-back pixmaps that DRI2
wants to work with.
gtkperf timing under mutter decreased from 21.99 to 14.45.