drm/i915: constify display wm vtable

Use a nop table for the cases where CxSR doesn't init properly.

v2: use a nop table (Jani)

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/bb0658d14afd02cca692cd58223800f68f4ff4ce.1632869550.git.jani.nikula@intel.com