drm/i915: Split out i915_gem_object_set_tiling()

Expose an interface for changing the tiling and stride on an object,
that includes the complexity of checking for conflicting bindings and
fence registers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170110121045.27144-2-chris@chris-wilson.co.uk