glx: make xf86vidmode mandatory for direct rendering

Currently we detect the module and if missing, the glXGetMsc* API is
effectively a stub, always returning false.

This is what effectively has been happening with our meson build :-(

Thus users have no chance of using it - they cannot even distinguish
if the failure is due to a misconfigured build.

There's no reason for keeping xf86vidmode optional - it has been
available in all distributions for years.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: a47c525f "meson: build glx"
(cherry picked from commit 5bc50936)