dca: support mixing LFE in Stereo downmix.

Embedded downmix coefficients can use this.
dcadec: treat all 2-channel modes as Stereo in dca_downmix.

The check for (prim_channels > 2) before calling dca_downmix made these cases unreachable, but now 2.1 layouts will go through the downmix code. Having dual mono, Lt/Rt and sum-difference layouts print errors when regular Stereo doesn't seems pointless.