drm/i915: Fix off-by-one in reporting hanging process

ffs() is 1-indexed, but we want to use it as an index into an array, so
use __ffs() instead.

Fixes: eb8d0f5a ("drm/i915: Remove GPU reset dependence on struct_mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315163933.19352-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1656,7 +1656,7 @@ error_msg(struct i915_gpu_state *error, unsigned long engines, const char *msg)
 			i915_error_generate_code(error, engines));
 	if (engines) {
 		/* Just show the first executing process, more is confusing */
-		i = ffs(engines);
+		i = __ffs(engines);
 		len += scnprintf(error->error_msg + len,
 				 sizeof(error->error_msg) - len,
 				 ", in %s [%d]",