avcodec: [loongson] fix bug of mss2-wmv failed in fate test.

Failed case: mss2-wmv
In following functions, pmullh was used to multiply two 16-bit data, this will cause data overflow.
1. ff_vc1_inv_trans_8x8_dc_mmi
2. ff_vc1_inv_trans_8x8_mmi
3. ff_vc1_inv_trans_8x4_mmi
4. ff_vc1_inv_trans_4x8_mmi
5. ff_vc1_inv_trans_4x4_mmi

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>