Export drmDevicesEqual

drmCompareBusInfo was almost this already, but it wasn't exported, its
name didn't match its functionality, and while it almost looks like it
was usable for sorting due to memcmp it wouldn't work if you had
multiple bus types. I don't really want to think about defining a
sensible sort order for bus types, so let's at least make it less of a
trap for the caller.

Invert its boolean sense to be 'true if equal', rename it to describe
the types it actually operates on, and export.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velilkov@collabora.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>