drm/i915: fix error return code in init_pipe_control()

Fix to return -ENOMEM in the kmap() error handling case
instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>