drm/i915: Simplify i915_gem_obj_is_pinned() test for set-tiling

Since the removal of the user pin_ioctl, the only means for pinning an
object is either through binding to the scanout or during execbuf
reservation. As the later prevents a call to set-tiling, we need only
check if the obj is pinned into the display plane to see if we need
reject the set-tiling ioctl.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>