drm/i915: Make WAIT_IOCTL negative timeouts be indefinite again

This fixes a regression from

commit 5ed0bdf2
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Wed Jul 16 21:05:06 2014 +0000

    drm: i915: Use nsec based interfaces

that made a negative timeout return immediately rather than the
previously defined behaviour of waiting indefinitely.

Testcase: igt/gem_wait
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89494


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: fixed a checkpatch complaint about whitespace.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>