mips/dspr2: fix vp9-highbitdepth build

vp9_itrans*_dspr2.c aren't necessary for high bitdepth builds and
notably vp9_itrans8_dspr2.c fails in various configurations using a
codesourcery toolchain:
vp9_itrans8_dspr2.c:31:5: can't find a register in class 'GR_REGS' while reloading 'asm'

Change-Id: I2ac76203e65cc643cb835ab50e95701896d92a1a
1 file changed