drm/i915: Introduce GEM object functions

GEM object functions deprecate several similar callback interfaces in
struct drm_driver. This patch replaces the per-driver callbacks with
per-instance callbacks in i915.

v2:
	* move object-function instance to i915_gem_object.c (Jani)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>