drm/i915: Squelch smatch warning for statement with no effect

The sgt iterators cause an

drivers/gpu/drm/i915/i915_gpu_error.c:846 i915_error_object_create() warn: statement has no effect 7

everywhere they are used. If we change the code slightly, we can achieve
the same increment without altering the output or raising a warning.

   text	   data	    bss	    dec	    hex	filename
1267906	  20587	   3168	1291661	 13b58d	before
1267906	  20587	   3168	1291661	 13b58d	after

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170913105754.4423-1-chris@chris-wilson.co.uk