drm/i915: constify display function vtable

Make nice clear tables instead of having things in two places.

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/d0f7e2c792dc40804555d26b6ede62f4bd2f23d6.1632869550.git.jani.nikula@intel.com