drm/i915: Reject set-tiling-ioctl with stride==0 and a tiling mode

In commit 957870f9 ("drm/i915: Split out i915_gem_object_set_tiling()"),
I swapped an alignment check for IS_ALIGNED and in the process removed
the less-than check. That check turns out to be important as it was the
only rejection for stride == 0. Tvrtko did spot it, but I was
overconfident in the IS_ALIGNED() conversion.

Fixes: 957870f9 ("drm/i915: Split out i915_gem_object_set_tiling()")
Testcase: igt/gem_tiling_max_stride
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170203105652.27819-1-chris@chris-wilson.co.ukReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem_tiling.c

@@ -173,7 +173,7 @@ i915_tiling_ok(struct drm_i915_gem_object *obj,
 	else
 		tile_width = 512;
 
-	if (!IS_ALIGNED(stride, tile_width))
+	if (!stride || !IS_ALIGNED(stride, tile_width))
 		return false;
 
 	/* 965+ just needs multiples of tile width */