drm/gma500: Fix error return code in psb_driver_load()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 5c49fd3a ("gma500: Add the core DRM files and headers")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130020216.1906141-1-zhangjialin11@huawei.com