RADEON/AIGLX DRI Lock At Initialization

[vehemens]

I believe I have narrowed the problem down to __glXDRIscreenProbe() removing 
the RADEON DRM lock that was set up with DRIFinishScreenInit().

What happens is that __glXDRIscreenProbe() calls drmOpenOnce() which uses 
drmAvailable() to test for the presence of the kernel driver.  If the test 
passes, it closes the file descriptor and removes the lock.

Without the lock, the DRI operations fail for DefineInitialRootWindow() and 
DRIFinishScreenInit() which run prior to DIspatch().   On my machine, this 
causes an OS hang.