drm/radeon: 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 radeon.

v2:
	* move object-function instance to radeon_gem.c (Christian)
	* set callbacks in radeon_gem_object_create() (Christian)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923102159.24084-14-tzimmermann@suse.de