drm/amd/amdgpu: Move vcn4 fw_shared init to a single function

- Move VCN4's fw_shared initialization to a separated function.
  This way, the function can be reused at different locations.

Signed-off-by: Bokun Zhang <bokun.zhang@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>