drm/i915/cnl: Allow 2 pixel per clock on Cannonlake.

This is heavily based on a initial patch provided by Ville
plus all changes provided later by Ander.

As Geminilake, Cannonlake also supports 2 pixels per clock.

Different from Geminilake we are not implementing the 99% Wa.
But we can revisit that decision later if we find out
any limitation on later CNL SKUs.

v2: Rebase on top of commit 'd305e061 ("drm/i915: Track
minimum acceptable cdclk instead of "minimum dotclock")'

v3: When fixing HDMI on CNL I noticed that I missed to convert
    back the doubled pixel rate to cdclk.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171003223142.26264-1-rodrigo.vivi@intel.com