vp9 does not support multi-res encoding, the request should not crash.

+ encode_api_test: unconditionally expose multi-res test

vpx_codec_enc_init_multi should fail independent of
CONFIG_MULTI_RES_ENCODING if not for the same reason.

Change-Id: I44fc58ef70ee4e0e482cb6a5736885f4cb2a8517
(cherry picked from commit 004fb914)