xf86drm: test for render nodes before primary nodes

Unlike Linux the OpenBSD primary "drm" device name is substring of the
"drmR" render node device name and strncmp() tests resulted in render
nodes being flagged as primary nodes.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>