drm/xen: 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 xen. The only exception is gem_prime_mmap,
which is non-trivial to convert.

v2:
	* convert xen_drm_drv_free_object_unlocked() to static
	  callback (Oleksandr)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>