glx: fixup symbol name for get_extensions function

glxProbeDriver() concatenates __DRI_DRIVER_GET_EXTENSIONS with driver name
to get symbol name for get_extension function. Unfortunately that doesn't
work for drivers that have hyphen in their name, e.g. sun4i-drm --
get_extensions() for these uses underscore instead.

As result dlsym() doesn't find get_extension() function and AIGLX
initialization fails resulting in following message in Xorg.0.log:

(EE) AIGLX error: sun4i-drm does not export required DRI extension

Replace all non-alpha-numeric characters with underscore to fix the issue.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>