drm/i915: Drop debugfs/i915_gem_pin_display

It has now lost its meaning (it shows more than just pin_display), I do
not believe that we are using in preference to the complete listing from
i915_gem_gtt, or the listing from i915_gem_framebuffer, or the listing
of active display objects in i915_display_info.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013202621.7276-3-chris@chris-wilson.co.uk