uxa/dri: Pixmap are offscreen and not attached to any display

uxa/dri: Pixmap are offscreen and not attached to any display

As the code current assumes that pixmaps are located at screen origin
(0,0), it assumes that they are all on the first CRTC. However, since
they are offscreen, this leads to such complications as attempting to
emit vsync swapbuffers, and even worse attempting to swap a GLXPixmap
onto the scanout following a DRI2SwapBuffers.

Reported-by: Alexander Monakov <amonakov@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73282
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>