minigbm: cros_gralloc: fix rendernode query

minigbm: cros_gralloc: fix rendernode query

If the vgem driver and a hardware driver are both present on
a machine, and if the vgem driver corresponds to a lower
number rendernode, we'll end up picking the vgem driver. This is
not the desired behavior, so let's fix it.

BUG=chromium:616275
TEST=mediatek driver loads
CQ-DEPEND=CL:416396

Change-Id: Ib2647f4dbaf3c45fab97bbbcbb94e09c1daa6d5d
Reviewed-on: https://chromium-review.googlesource.com/416397
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>