drm/via: Use SPDX tag for MIT license in via_3d_reg header

The license for the via_3d_reg header is MIT - so use the
shorter SPDX tag to identify the license.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Kevin Brace <kevinbrace@bracecomputerlab.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220713170202.1798216-12-sam@ravnborg.org