update registers header
add libdrm debugging/traces

Normally this gets registered in xserver DRI1 code, but we don't support
DRI1.
exa: use CreatePixmap2

This will be handy later if we want DRI to request creation of render
targets in SMI.
exa: add support for composite w/ repeat
accel: reduce cmdstream buffer size

The size field in VGV3_NEXTCMD is only 12 bits, so we can submit
cmdstream packets with payload larger than this.  Reduce the size
of the cmdstream buffer to compensate.

Also, the smaller size turns up some bugs in handling of ringbuffer
switch (we'd switch in BEGIN_RING() -> FIRE_RING(), but the exa fxns
grab the ringbuffer ptr before that), so fix that as well.