drm/i915/icl: Add allowed DP rates for Icelake

drm/i915/icl: Add allowed DP rates for Icelake

For ICL, on Combo PHY the allowed max rates are:
 - HBR3 8.1 eDP (DDIA)
 - HBR2 5.4 DisplayPort (DDIB)
and for MG PHY/TC DDI Ports allowed DP rates are:
 - HBR3 8.1 DisplayPort (DP alternate mode, DP over TBT,
 - DP on legacy connector - DDIC/D/E/F)

v2 (from Paulo): Remove misleading comment (Ville).

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
[Paulo: bikeshed to keep future platforms on "else", v2.]
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180611222655.5696-1-paulo.r.zanoni@intel.com