drm/vmwgfx: Add SPDX header to vmwgfx_drm.h

Update vmwgfx_drm.h with SPDX-License-Identifier:
(GPL-2.0 WITH Linux-syscall-note) OR MIT

Signed-off-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240126200804.732454-4-zack.rusin@broadcom.com