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

drivers/gpu/drm/i915/gt/intel_ring_submission.c

@@ -1265,7 +1265,7 @@ static struct i915_vma *gen7_ctx_vma(struct intel_engine_cs *engine)
 	int size, err;
 
 	if (GRAPHICS_VER(engine->i915) != 7 || engine->class != RENDER_CLASS)
-		return 0;
+		return NULL;
 
 	err = gen7_ctx_switch_bb_setup(engine, NULL /* probe size */);
 	if (err < 0)