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

v3:
	* remove amdgpu_object.c from patch (Christian)
v2:
	* move object-function instance to amdgpu_gem.c (Christian)
	* set callbacks in amdgpu_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/20200923102159.24084-2-tzimmermann@suse.de