drm/gma500: 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.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: c2f17e60 ("drm/gma500: Embed struct drm_device in struct drm_psb_private")
Reported-by: Hulk Robot <hulkci@huawei.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924094040.3631675-1-yangyingliang@huawei.com