Move display watermark related members under drm_i915_private display
sub-struct.

It's a bit arbitrary when to define a named struct for grouping, but
clearly intel_wm is big enough to warrant a separate definition.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b991dd25f8f539746c475d417691e0816ff6b24a.1661346845.git.jani.nikula@intel.com