drm/i915: split watermark vfuncs from display vtable.

These are the watermark api between display and pm.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7130356324ef3de59b4e913f025d7dce822157ee.1632869550.git.jani.nikula@intel.com