drm/i915: Simplify skl_max_scale()

Now that the planes declare their minimum cdclk requirements properly
we don't need to check the cdclk in skl_max_scale() anymore. Just check
against the maximum downscale ratio, and move the code next to it's
only caller.

v2: Add a comment explaining the HQ vs. not thing

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015193035.25982-7-ville.syrjala@linux.intel.com