drm/vmwgfx: add SPDX idenitifier and clarify license

These files are licensed under GPL-2.0.
Removing the MIT boilerplate as that really didn't make any sense for
those two header files.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dirk Hohndel (VMware) <dirk@hohndel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180506231626.115996-8-dirk@hohndel.org