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: e8529dbc75 ("drm/amdgpu: add ip offset support for cyan skillfish")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4654
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: MIT
 /*
  * Copyright 2018 Advanced Micro Devices, Inc.
  *