drm/vram: Set GEM object functions for PRIME

PRIME functionality is now provided via the callback functions in
struct drm_gem_object_funcs. The driver-structure functions are obsolete.
As a side effect of this patch, VRAM-based drivers get basic PRIME
support automatically without having to set any flags or additional
fields.

v2:
	- use existing PRIME functions for object's table
v3:
	- move object table to EOF so it can refer to internal interfaces

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702115012.4418-2-tzimmermann@suse.de