drm/i915: Check the timeout passed to i915_wait_request

We have relied upon the sole caller (wait_ioctl) validating the timeout
argument. However, when waiting for multiple requests I forgot to ensure
that the timeout was still positive on the later requests. This is more
simply done inside __i915_wait_request.

Fixes regression introduced in
commit b4716185


Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Apr 27 13:41:17 2015 +0100

    drm/i915: Implement inter-engine read-read optimisations

The impact of the regression is 1 jiffie for each extra active ring for
a wait_ioctl with a timeout -- I don't think anyone has noticed.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1448544702-5594-1-git-send-email-chris@chris-wilson.co.uk
Signed-off-by: Jani Nikula <jani.nikula@intel.com>