Disable usage of ff_h264_idct_add_altivec since AltiVec versions of h264_idct_add16,

h264_idct_add16intra, h264_idct_add8 need to be implemented.

Add C version of ff_h264_idct8_dc_add in AltiVec so that ff_h264_idct8_add_altivec
can be used.

Originally committed as revision 16311 to svn://svn.ffmpeg.org/ffmpeg/trunk