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

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 <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Dhinakaran Pandiyan <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Reviewed-by: Dhinakaran Pandiyan <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]