drm/i915/gvt: Fix one indent error

Fix below warning:

drivers/gpu/drm/i915//gvt/handlers.c:323 gdrst_mmio_write() warn: inconsistent indenting
Reviewed-by: NZhi Wang <zhi.a.wang@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>