drm/amdgpu/debugfs: fix memory leak when pm_runtime_get_sync failed

Fix memory leak in amdgpu_debugfs_gpr_read not freeing data when
pm_runtime_get_sync failed.

Fixes: a9ffe2a9 ("drm/amdgpu/debugfs: properly handle runtime pm")
Signed-off-by: Chen Tao <chentao107@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>