Moving pass from VP9_COMP to VP9EncoderConfig.

We had a very complicated way to initialize cpi->pass from
cfg->g_pass:
switch (cfg->g_pass) {
  case VPX_RC_ONE_PASS:
    oxcf->mode = ONE_PASS_GOOD;
    break;
  case VPX_RC_FIRST_PASS:
    oxcf->mode = TWO_PASS_FIRST;
    break;
  case VPX_RC_LAST_PASS:
    oxcf->mode = TWO_PASS_SECOND_BEST;
    break;
}

cpi->pass = get_pass(oxcf->mode).

Now pass is moved to VP9EncoderConfig and initialization is simple:
switch (cfg->g_pass) {
  case VPX_RC_ONE_PASS:
    oxcf->pass = 0;
    break;
  case VPX_RC_FIRST_PASS:
    oxcf->pass = 1;
    break;
  case VPX_RC_LAST_PASS:
    oxcf->pass = 2;
    break;
}

Change-Id: I8f582203a4575f5e39b071598484a8ad2b72e0d9