drm/i915: move wm_disp funcs to display.funcs

Move display watermark functions under drm_i915_private display
sub-struct.

Rename struct drm_i915_wm_disp_funcs to intel_wm_funcs while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/29d07c82ef7d33a59fc9c8e25ae2d2f900677a4c.1661346845.git.jani.nikula@intel.com