drm/rockchip: Use drm_gem_fb_create_with_dirty

If rockchip would switch over to the generic fbdev setup we could
grabage collect even more of all this code (all of the remaining fb
handling code really).

v2: Actually use _with_dirty like the patch subject promised (Andrzej)

Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191127180035.416209-1-daniel.vetter@ffwll.ch