drm/i915: fix return value check of debugfs_create_file()

In case of error, the function debugfs_create_file() returns NULL
pointer not ERR_PTR() if debugfs is enabled. The IS_ERR() test in
the return value check should be replaced with NULL test.

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