drm/radeon: Fix eDP for single-display iMac10,1 (v2)

The late 2009, 27 inch Apple iMac10,1 has an
internal eDP display and an external Mini-
Displayport output, driven by a DCE-3.2, RV730
Radeon Mobility HD-4670.

The machine worked fine in a dual-display setup
with eDP panel + externally connected HDMI
or DVI-D digital display sink, connected via
MiniDP to DVI or HDMI adapter.

However, booting the machine single-display with
only eDP panel results in a completely black
display - even backlight powering off, as soon as
the radeon modesetting driver loads.

This patch fixes the single dispay eDP case by
assigning encoders based on dig->linkb, similar
to DCE-4+. While this should not be generally
necessary (Alex: "...atom on normal boards
should be able to handle any mapping."), Apple
seems to use some special routing here.

One remaining problem not solved by this patch
is that an external Minidisplayport->DP sink
does still not work on iMac10,1, whereas external
DVI and HDMI sinks continue to work.

The problem affects at least all tested kernels
since Linux 3.13 - didn't test earlier kernels, so
backporting to stable probably makes sense.

v2: With the original patch from 2016, Alex was worried it
    will break other DCE3.2 systems. Use dmi_match() to
    apply this special encoder assignment only for the
    Apple iMac 10,1 from late 2009.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>