drm/i915/gt: Initialise rps timestamp

Smatch warns that we may iterate over an empty array of gt->engines[].
One hopes that this is impossible, but nevertheless we can simply
appease smatch by initialising the timestamp to zero before we starting
probing the busy-time from the engines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200619151938.21740-1-chris@chris-wilson.co.uk