drm/i915/selftests: Silence compiler warning in igt_ctx_exec

The compiler doesn't always spot the guard that object is allocated on
the first pass, leading to:

drivers/gpu/drm/i915/selftests/i915_gem_context.c: warning: 'obj' may be used uninitialized in this function [-Wuninitialized]:  => 370:8

v2: Make it more obvious by setting obj to NULL on the first pass and
any later pass where we need to reallocate.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: 791ff39ae3 ("drm/i915: Live testing for context execution")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
c: <drm-intel-fixes@lists.freedesktop.org> # v4.12-rc1+
Link: http://patchwork.freedesktop.org/patch/msgid/20170523194412.1195-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/selftests/i915_gem_context.c

@@ -320,7 +320,7 @@ static unsigned long max_dwords(struct drm_i915_gem_object *obj)
 static int igt_ctx_exec(void *arg)
 {
 	struct drm_i915_private *i915 = arg;
-	struct drm_i915_gem_object *obj;
+	struct drm_i915_gem_object *obj = NULL;
 	struct drm_file *file;
 	IGT_TIMEOUT(end_time);
 	LIST_HEAD(objects);
@@ -359,7 +359,7 @@ static int igt_ctx_exec(void *arg)
 		}
 
 		for_each_engine(engine, i915, id) {
-			if (dw == 0) {
+			if (!obj) {
 				obj = create_test_object(ctx, file, &objects);
 				if (IS_ERR(obj)) {
 					err = PTR_ERR(obj);
@@ -376,8 +376,10 @@ static int igt_ctx_exec(void *arg)
 				goto out_unlock;
 			}
 
-			if (++dw == max_dwords(obj))
+			if (++dw == max_dwords(obj)) {
+				obj = NULL;
 				dw = 0;
+			}
 			ndwords++;
 		}
 		ncontexts++;