drm/i915/dp: Link train Fallback on eDP only if fallback link BW can fit panel's...

drm/i915/dp: Link train Fallback on eDP only if fallback link BW can fit panel's native mode

This patch fixes the original commit c0cfb10d9e1de49 ("drm/i915/edp:
Do not do link training fallback or prune modes on EDP") that causes
a blank screen in case of certain eDP panels (Eg: seen on Dell XPS13 9350)
where first link training fails and a retraining is required by falling
back to lower link rate/lane count.
In case of some panels they advertise higher link rate/lane count
than whats required for supporting the panel's native mode.
But we always link train at highest link rate/lane count for eDP
and if that fails we can still fallback to lower link rate/lane count
as long as the fallback link BW still fits the native mode to avoid
pruning the panel's native mode yet retraining at fallback values
to recover from a blank screen.

v3:
* Add const for fixed_mode (Ville)
v2:
* Send uevent if link failure on eDP unconditionally

Fixes: c0cfb10d9e1d ("drm/i915/edp: Do not do link training fallback or prune modes on EDP")
Cc: Clinton Taylor <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Ville Syrjala <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Lucas De Marchi <[email protected]>
Cc: <[email protected]> # v4.17+
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107489
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105338
Signed-off-by: Manasi Navare <[email protected]>
Tested-by: Alexander Wilson <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]