drm/amdgpu: fix SPDX header on cyan_skillfish_reg_init.c

This should be MIT. The driver in general is MIT and
the license text at the top of the file is MIT so fix
it.

Fixes: e8529dbc75ca ("drm/amdgpu: add ip offset support for cyan skillfish")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4654
Reviewed-by: Mario Limonciello <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit 102c4f7c554ac5a5ecf0023fa0612beb58e3b0bd)

drivers/gpu/drm/amd/amdgpu/cyan_skillfish_reg_init.c

···
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: MIT
 /*
  * Copyright 2018 Advanced Micro Devices, Inc.
  *