drm/i915: s/0/NULL/

Use NULL where appropriate.

drivers/gpu/drm/i915/gt/intel_ring_submission.c:1210:24: warning: Using plain integer as NULL pointer

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712161815.24776-2-ville.syrjala@linux.intel.com


Reviewed-by: Matthew Auld <matthew.auld@intel.com>