drm/i915: Track minimum acceptable cdclk instead of "minimum dotclock"

Make the min_pixclk thing less confusing by changing it to track
the minimum acceptable cdclk frequency instead. This means moving
the application of the guardbands to a slightly higher level from
the low level platform specific calc_cdclk() functions.

The immediate benefit is elimination of the confusing 2x factors
on GLK/CNL+ in the audio workarounds (which stems from the fact
that the pipes produce two pixels per clock).

v2: Keep cdclk higher on CNL to workaround missing DDI clock voltage handling
v3: Squash with the CNL cdclk limits patch (DK)
v4: s/intel_min_cdclk/intel_pixel_rate_to_cdclk/ (DK)

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170830185703.8189-1-ville.syrjala@linux.intel.com