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