drm/i915/ringbuffer: change header SPDX identifier to MIT

Commit b2441318 ("License cleanup: add SPDX GPL-2.0 license
identifier to files with no license") added "SPDX-License-Identifier:
GPL-2.0" to files which previously had no license, change this to MIT
for intel_ringbuffer.h matching the license text of intel_ringbuffer.c.
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031005331.20775-1-jsg@jsg.id.au

drivers/gpu/drm/i915/intel_ringbuffer.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: MIT */
 #ifndef _INTEL_RINGBUFFER_H_
 #define _INTEL_RINGBUFFER_H_