drm/hisilicon/hibmc: Fix wrong pointer passed to PTR_ERR()

PTR_ERR should access the value just tested by IS_ERR, otherwise
the wrong error code will be returned.

Fixes: d1667b86 ("drm/hisilicon/hibmc: Add support for frame buffer")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170112151921.16538-1-weiyj.lk@gmail.com