drm/amdgpu/gfx9: switch to amdgpu_gfx_rlc_init_microcode

switch to common helper to initialize rlc firmware
for gfx9

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>