drm: uapi: Use SPDX in DRM core uAPI headers

drm: uapi: Use SPDX in DRM core uAPI headers

The DRM core uAPI headers are licensed under the MIT license, and carry
copies of the license with slight variations. Replace them with SPDX
headers.

Following a discussion with Simona Vetter on this topic, add a
clarification in the drm-uapi.rst file that independent closed-source
userspace implementations of software using the DRM uAPI are accepted,
as allowed by the MIT license.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patch.msgid.link/20260407104951.1781047-1-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>