drm/i915: Mark i915.reset as unsigned

We have been using '-1' to mean the maximum i915.reset level in the
belief that it was unsigned... This turns out to have been a grave
error, and instead of setting the default reset mechanism in igt,
we have been disabling reset!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200205123757.1834947-1-chris@chris-wilson.co.uk