glx: Fix number of property values to read in glXImportContextEXT

glx: Fix number of property values to read in glXImportContextEXT

We were trying to read twice as many as the X server sent us, which
upset XCB:

[xcb] Too much data requested from _XRead
[xcb] This is most likely caused by a broken X extension library
[xcb] Aborting, sorry about that.
glx-free-context: ../../src/xcb_io.c:732: _XRead: Assertion `!xcb_xlib_too_much_data_requested' failed.

Fixing this takes 3 GLX piglit tests from crash to pass.

Fixes: 085216295033 "glx: Be more tolerant in glXImportContext (v2)"
Reviewed-by: Adam Jackson <ajax@redhat.com>