drm/i915/dp: optimize eDP 1.4+ link config fast and narrow

drm/i915/dp: optimize eDP 1.4+ link config fast and narrow

We've opted to use the maximum link rate and lane count for eDP panels,
because typically the maximum supported configuration reported by the
panel has matched the native resolution requirements of the panel, and
optimizing the link has lead to problems.

With eDP 1.4 rate select method and DSC features, this is decreasingly
the case. There's a need to optimize the link parameters. Moreover,
already eDP 1.3 states fast link with fewer lanes is preferred over the
wide and slow. (Wide and slow should still be more reliable for longer
cable lengths.)

Additionally, there have been reports of panels failing on arbitrary
link configurations, although arguably all configurations they claim to
support should work.

Optimize eDP 1.4+ link config fast and narrow.

Side note: The implementation has a near duplicate of the link config
function, with just the two inner for loops turned inside out. Perhaps
there'd be a way to make this, say, more table driven to reduce the
duplication, but seems like that would lead to duplication in the table
generation. We'll also have to see how the link config optimization for
DSC turns out.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: "Lee, Shawn C" <shawn.c.lee@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105267
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905095321.13843-1-jani.nikula@intel.com