drm/i915: Update DRIVER_DATE to 20180921

#define DRIVER_NAME		"i915"

#define DRIVER_DESC		"Intel Graphics"

#define DRIVER_DATE		"20180906"

#define DRIVER_TIMESTAMP	1536242083

#define DRIVER_DATE		"20180921"

#define DRIVER_TIMESTAMP	1537514460

/* Use I915_STATE_WARN(x) and I915_STATE_WARN_ON() (rather than WARN() and

 * WARN_ON()) for hw state sanity checks to check for unexpected conditions