drm/amdgpu: optimize gfx9 init_microcode function

Split each type of firmware into single function for easy to maintain.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>