Support for "control" nodes was removed from the drm subsystem some time

Support for "control" nodes was removed from the drm subsystem some time
ago, but some code in drmopen() remained which means that opening a drm
device node with a minor that matches the range for the "control" nodes
will hit a kernel assertion.  A similar issue exists for "render" nodes
corresponding to a driver that only supports KMS (such as rkdrm(4)).

Add checks to see if the minor is valid and return ENXIO if that isn't the
case to prevent a kernel crash.

ok jsg@, miod@