drm/i915: don't allow tiling changes on pinned buffers v2

Makes no sense and complicates matters for pipelined tiling changes.
So don't allow it and return -EBUSY.

v2: Fix reference leak. Thanks to Owain Ainsworth for spotting this.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>