drm/i915/trace: i915_request.prio is a signed value

Don't confuse the poor developer by writing a negative value as a very
large positive, as the flow of requests is already complex enough.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128151647.3820659-1-chris@chris-wilson.co.uk