drm/i915: fix return value check of debugfs_create_file()
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 <b8f9cab8be13de37b9588aedad10a20fc3a68783@trendmicro.com.cn>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs