drm/amdgpu: add function to show psp_gfx_cmd name via id

Implement function psp_gfx_cmd_name to show cmd name
via cmd id.

v2: rename it to psp_gfx_cmd_name

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>