drm/i915: Use REG_FIELD_GET() to extract skl+ wm latencies

Replace the hand rolled stuff with REG_FIELD_GET() for reading
out the skl+ watermark latencies.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908191646.20239-4-ville.syrjala@linux.intel.com


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