drm/i915: error_buffer->ring should be signed

gcc seems to get uber-anal recently about these things.

Clarification from Dan Carpenter:

"Sorry, I should have said that it's not a gcc warning, it's a smatch
thing.  But also it's not uber-anal.  It's the exact level of anality
which is required to make the == -1 test work.  You can compare
unsigned int and longs to -1 and it works but for smaller types it
doesn't."
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -199,7 +199,7 @@ struct drm_i915_error_state {
 		u32 tiling:2;
 		u32 dirty:1;
 		u32 purgeable:1;
-		u32 ring:4;
+		s32 ring:4;
 		u32 cache_level:2;
 	} *active_bo, *pinned_bo;
 	u32 active_bo_count, pinned_bo_count;