Fix assertion failures in mips+msa setting

Directly call c functions, otherwise when EXT_TX is enabled, hybrid
transform other than combination of DCT/ADST has not been implemented, thus
will cause assertion failures in the switch loops in vp10_fhtnxn_msa() and
vp10_ihtnxn_nxn_add_msa().

BUG=webm:1239

Change-Id: I2379a07e5406f9489edcd2f3205682f679c9b091