vp9: add itxfm_add eob shortcuts to 10/12bpp functions.

These aren't quite as helpful as the ones in 8bpp, since over there,
we can use pmulhrsw, but here the coefficients have too many bits to
be able to take advantage of pmulhrsw. However, we can still skip
cols for which all coefs are 0, and instead just zero the input data
for the row itx. This helps a few % on overall decoding speed.

tests/checkasm/vp9dsp.c

@@ -337,7 +337,7 @@ static void check_itxfm(void)
                     randomize_buffers();
                     ftx(coef, tx, txtp, sz, bit_depth);
 
-                    for (sub = (txtp == 0) ? 1 : sz; sub <= sz; sub <<= 1) {
+                    for (sub = (txtp == 0) ? 1 : 2; sub <= sz; sub <<= 1) {
                         int eob;
 
                         if (sub < sz) {