drm/i915: Remove unused to_gem_object() macro

drm/i915: Remove unused to_gem_object() macro

That macro was only ever used to convert ring->private into a gem object
(hence the forceful cast). ring->private doesn't even exist anymore as
it was transmogrified by Chris in:

  commit 0d1aacac36530fce058d7a0db3da7befd5765417
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Mon Aug 26 20:58:11 2013 +0100

      drm/i915: Embed the ring->private within the struct intel_ring_buffer

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>