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

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

Commit e18caa17255e ("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