drm/driver: Add defaults for .gem_prime_export/import callbacks

The majority of drivers use drm_gem_prime_export() and
drm_gem_prime_import() for these callbacks so let's make them the
default.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110145647.17580-2-noralf@tronnes.org