drm/amd/pp: Implement load_firmware interface

with this interface, gfx/sdma can be initialized
before smu.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>