drm/amdgpu: fix compiler warning for amdgpu_gfx_cp_init_microcode

Change the type of parameter on amdgpu_gfx_cp_init_microcode to fix
compiler warning.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>