drm/i915/wm: remove display/ prefix from include

Remove the leftover from moving and renaming the file from driver top
level.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f11cbbdb5a5c8961fcae0b3f6c87860ee00f8c26.1677678803.git.jani.nikula@intel.com