drm/virtgpu: Set PRIME export function in struct drm_gem_object_funcs

GEM object functions deprecate several similar callback interfaces in
struct drm_driver. This patch replaces virtgpu's per-driver PRIME export
function with a per-object function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923102159.24084-19-tzimmermann@suse.de