gbm: try to open a render node before the card device

Since render nodes allow rendering which should not interact with
the window system, we attempt to use render nodes first when a gbm
device is not directly specified.

The main difference as far as gbm is concerned is that render
nodes don't require drm master authentication.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>