sna: Actually reuse pixmap headers for ordinary pixmaps

sna: Actually reuse pixmap headers for ordinary pixmaps

commit 3dbf17f00e200e864818b0b8ca762183cff9085f [2.99.907]
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Dec 3 12:10:01 2013 +0000

    sna: Mark up an ordinary pixmap for reuse

started to put the frequently allocated pixmaps into the object cache -
but as those frequent allocations themselves did not use the cache, the
cache kept growing.

Bugzilla: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1272338
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>