Documentation/i915: fix file references after display/ subdir renames

Fix the plethora of Sphinx build errors after moving the display files
under a subdirectory.

Fixes: 379bc100 ("drm/i915: move modesetting output/encoder code under display/")
Fixes: df0566a6 ("drm/i915: move modesetting core code under display/")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617102944.25129-1-jani.nikula@intel.com