We thought that no userspace is using them, but oops libdrm is using
them to figure out whether a driver supports modesetting. Check out
drmCheckModesettingSupported but maybe don't because it's horrible and
totally runs counter to where we want to go with libdrm device
handling. The function looks in the device hierarchy for whether
controlD* exist using the following format string:

/sys/bus/pci/devices/%04x:%02x:%02x.%d/drm/controlD%d

The "/drm" subdirectory is the glue directory from the sysfs class
stuff, and the only way to get at it seems to through
kdev->kobj.parent (when kdev is represents e.g. the card0 chardev
instance in sysfs). Git grep says we're not the only ones touching
that, so I hope it's ok we dig into such internals - I couldn't find a
proper interface for getting at the glue directory.

Quick git grep shows that at least -amdgpu, -ati are using this.
-modesetting do not, and on -intel it's only about the 4th fallback
path for device lookup, which is why this didn't blow up earlier.

Oh well, we need to keep it working, and the simplest way is to add a
symlink at the right place in sysfs from controlD* to card*.

v2:
- Fix error path handling by adding if (!minor) return checks (David)
- Fix the controlD* numbers to match what's been there (David)
- Add a comment what exactly userspace minimally needs.
- Correct the analysis for -intel (Chris).

Fixes: 8a357d10 ("drm: Nerf DRM_CONTROL nodes")
Cc: Dave Airlie <airlied@gmail.com>
Reported-and-tested-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161209135656.14881-1-daniel.vetter@ffwll.ch