idct.c tweaks.
idct.c tweaks.

- Clarify comment about using C version.
- Call general versions instead of C version. No impact for now, as
  assembly versions of these don't exist. But this will avoid any
  future mistakes.

Change-Id: Iccb9e57d5a8843d6dffc1cf9b3164efbd25919e7

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom