xf86: Accept devices with the 'simpledrm' driver.

SimpleDRM 'devices' are a fallback device, and do not have a busid
so they are getting skipped. This will allow simpledrm to work
with the modesetting driver

(cherry picked from commit b9218fad)
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>