egl: only take render nodes into account when listing DRM devices

We don't want to expose an EGL device for a display-only DRM devices
(like VKMS). For these DRM devices we have a separate software-rendering
device (the first in the list, always present).

There is a similar check in _eglAddDRMDevice, however it will be
removed in a future commit to allow split render/display devices
to be properly added. We can't figure out whether we're on a split
render/display system before loading the driver.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 5743a36b ("egl: Don't add hardware device if there is no render node v2.")
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!9697>
(cherry picked from commit e39d72ae)