drm: Fix error handling in drm_legacy_addctx

'ctx->handle' is unsigned, it never less than zero.
This patch use int 'tmp_handle' to handle the err condition.

Fixes: 62968144 ("drm: convert drm context code to use Linux idr")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181229024907.12852-1-yuehaibing@huawei.com