glx: mandate xf86vidmode only for "drm" dri platforms

Currently we have the three dri "platforms" - drm, apple and windows.

Since xf86vidmode is a thing only for the drm one, adjust the
preprocessor guards and correctly check for the dependency.

v2: terminate the GLX_USE_WINDOWSGL hunk

Cc: Jon TURNEY <jon.turney@dronecode.org.uk>
Fixes: 5bc50936 ("glx: make xf86vidmode mandatory for direct rendering")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit a95ec138)