drm/i915: Copy the plane hw state directly for Y planes

When doing the plane state copy from the UV plane to the Y plane
let's just copy the hw state directly instead of using the original
uapi state. The UV plane has already had its uapi state copied into
its hw state, so this extra detour via the uapi state for the Y plane
is pointless.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-2-manasi.d.navare@intel.com