configure: EGL requirements only apply if EGL is built

Issue was hit with this configuration:
  --disable-{egl,gbm} --with-platform=drm

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 3208fd2e ("configure: move platform handling further up")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 610f956f)