drm/i915: Add missing HDMI audio pixel clocks for gen12

Gen12 hardware supports HDMI audio pixel clocks of 296.7/297Mhz
and 593.4/594Mhz. Add the missing rates and add logic to ignore
them if running on older hardware.

Bspec: 49333
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310162338.9387-1-kai.vehmanen@linux.intel.com