drm/i915/gt: Use memset_p to clear the ports

Put memset_p to use to clear the array of pointers used for tracking the
ELSP.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106114234.2529613-6-chris@chris-wilson.co.uk