drm/amdgpu: unlock mutex on error

Make sure to unlock the mutex when error happen

v2:
1. correct syntax error in the commit comments
2. remove change-Id

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>