drm/i915/wm: move functions to call watermark hooks to intel_wm.[ch]

Move the wrappers to call watermark hooks into intel_wm.[ch]. This
declutters intel_display.c nicely.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2c8243c5c81b8cd8e34d51f55f3533373c305d0e.1676317696.git.jani.nikula@intel.com