drm/i915: Reuse ilk_needs_fb_cb_tune() for the reduced clock as well

Use ilk_needs_fb_cb_tune() for reduced_clock instead of hand rolling
it. Also ilk_needs_fb_cb_tune() can just use the precomputed M value
instead of calculating it again.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210715093530.31711-11-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>