Objects can have multiple VMAs used for display in which
case assertion that objects must not be pinned for display
more times than the current VMA is incorrect.

v2: Commit message update. (Chris Wilson)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 058d88c4 ("drm/i915: Track pinned VMA")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1477413635-3876-1-git-send-email-tvrtko.ursulin@linux.intel.com