glx: Fix build in CMAKE_BUILD_TYPE=Debug

Fix this error.

glx_context.c: In function ‘glx_context_fill_attrib_list’:
glx_context.c:112:24: error: ‘dpy’ undeclared
  assert(dpy->ARB_create_context_profile);

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>