drm/rockchip: Use .dumb_map_offset and .dumb_destroy defaults

This driver can use the drm_driver.dumb_destroy and
drm_driver.dumb_map_offset defaults, so no need to set them.

Cc: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500837417-40580-25-git-send-email-noralf@tronnes.org