drm/rockchip: Drop unbalanced obj unref

In the error path, rockchip_drm_gem_object_mmap() is dropping an obj
reference that it doesn't own.

Fixes: 41315b79 ("drm/rockchip: use drm_gem_mmap helpers")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230119231734.2884543-1-robdclark@gmail.com