glx: Move DRI extensions pointer loading to driOpenDriver().

glx: Move DRI extensions pointer loading to driOpenDriver().

The only thing you do with a dri driver handle is get the extensions
pointer, so just fold it in to simplify the callers.

v2: Add the declaration of driGetDriverExtensions() that got lost in a
    rebase.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)