glx: fix xshm check to init xshm_opcode.
Found and proposed by Ray Strode (halfline)
Fixes: 68e89401 ("glx/drisw: use xcb instead of X to query connection")
Found and proposed by Ray Strode (halfline)
Fixes: 68e89401 ("glx/drisw: use xcb instead of X to query connection")