drm/amdgpu: add wait_for helper for spirom update

Spirom update typically requires extremely long
duration for command execution, and special helper
function to wait for it completion.

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>