encoder: remove postproc dependency

Remove the dependency on postproc.c for the encoder in general, the only
unchecked need for it is when CONFIG_PSNR is enabled. All other cases
are already wrapped in CONFIG_POSTPROC. In the CONFIG_PSNR case the file
will still be included.

Additionally, when VP8_SET_POSTPROC is used with the encoder when post
processing has been disabled an error will be returned.

This addresses issue #153.

Change-Id: Ia6dfe20167f7077734a6058cbd1d794550346089