drm/i915: Remove two invalid warns

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