drm/i915: Polish ilk+ wm register bits

Use REG_GENMASK() & co. for ilk+ watermark registers.

v2: Stick to the current bitmask sizes (Jani)
    Fix "watermarm" typo (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220216232806.6194-4-ville.syrjala@linux.intel.com