drm/i915/cnl: Add allowed DP rates for Cannonlake.

"Frequencies over 5.4 GHz only supported on certain
DDI ports and SKUs, and requires Vccio >= 0.95V."

More specifically, for current CNL SKUs available
(CNL-U and CNL-Y) we have:

DDI A - 5.4G eDP
DDI B - 8.1G DP
DDI C - 8.1G DP
DDI D - 5.4G DP

v2: Rebase on top of source_rates changes.
v3: Address the max 5.4 x 8.1 per DDI and also consider vccio.

Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170810224008.15571-1-rodrigo.vivi@intel.com