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.

v2:
- allow only bw values explicitly listed in the DP standard (Daniel,
  Chris)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>