drm/amdgpu: correct SPDX identifier in amdgpu_trace_points.c

Commit b2441318


'License cleanup: add SPDX GPL-2.0 license identifier to files with no license'
incorrectly added "SPDX-License-Identifier: GPL-2.0" to a file with MIT
license text.  Change the SPDX identifier to match the license text.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>