drm/i915: Make .modeset_calc_cdclk() mandatory

drm/i915: Make .modeset_calc_cdclk() mandatory

While not all platforms allow us to change the cdclk frequency
we should still verify that the fixed cdclk frequency isn't
too low. To that end let's cook up a .modeset_calc_cdclk()
implementation that only does the min_cdclk vs. actual cdclk
frequency check for such platforms.

Also we mustn't forget about double wide pipe on gen2/3 when
doing this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708125325.16576-11-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>