drm/amdgpu: factor out the AMDGPU_INFO_FW_VERSION case branch into amdgpu_firmware_info

The new amdgpu_firmware_info function will be used on amdgpu firmware
version debugfs.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>