drm/i915: Focus debugfs/i915_gem_pinned to show only display pins

Only those objects pinned to the display have semi-permanent pins of a
global nature (other pins are transient within their local vm). Simplify
i915_gem_pinned to only show the pertinent information about the pinned
objects within the GGTT.

v2: i915_gem_gtt_info is still shared with debugfs/i915_gem_gtt,
rename i915_gem_pinned to i915_gem_pin_display to better reflect its
contents

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-5-git-send-email-chris@chris-wilson.co.uk