drm/i915: Use __printf markup to silence compiler

drm/i915: Use __printf markup to silence compiler

drivers/gpu/drm/i915/i915_gpu_error.c: In function ‘i915_error_vprintf’:
drivers/gpu/drm/i915/i915_gpu_error.c:137:3: warning:
function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
   len = vsnprintf(NULL, 0, f, tmp);
   ^~~
drivers/gpu/drm/i915/i915_gpu_error.c:144:2: warning:
function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
  len = vsnprintf(e->buf + e->bytes, e->size - e->bytes, f, args);
  ^~~

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170114105113.1231-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>