drm/gma500: Add the missed drm_gem_object_put() in psb_user_framebuffer_create()

psb_user_framebuffer_create() misses to call drm_gem_object_put() in an
error path. Add the missed function call to fix it.

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210629115956.15160-1-jingxiangfeng@huawei.com