drm/i915/dp: Use slow and wide link training for everything

drm/i915/dp: Use slow and wide link training for everything

Screen flickers on Innolux eDP 1.3 panel when clock rate 540000 is in use.

According to the panel vendor, though clock rate 540000 is advertised,
but the max clock rate it really supports is 270000.

Ville Syrjälä mentioned that fast and narrow also breaks some eDP 1.4
panel, so use slow and wide training for all panels to resolve the
issue.

User also confirmed that the new strategy doesn't introduce any
regression on XPS 9380.

v2:
 - Use slow and wide for everything.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3384
References: https://gitlab.freedesktop.org/drm/intel/-/issues/272
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421052054.1434718-1-kai.heng.feng@canonical.com