drm/i915: fix lane bandwidth capping for DP 1.2 sinks
drm/i915: fix lane bandwidth capping for DP 1.2 sinks

DP 1.2 compatible displays may report a 5.4Gbps maximum bandwidth which
the driver will treat as an invalid value and use 1.62Gbps instead. Fix
this by capping to 2.7Gbps for sinks reporting a 5.4Gbps max bw.

Also add a warning for reserved values.

From Imre Deak
d4eead50eb206b875f54f66cc0f6ec7d54122c28 in mainline linux

One of two backported commits to prevent a black screen
on a Lenovo IdeaPad Yoga 2 Pro as reported/debugged by Steven Wilson.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars