vp8 encoder parameter check - do not check for ref frames if no no_ref_X flag is set

vp8 encoder parameter check - do not check for ref frames if no no_ref_X flag is set

In VP8 encoding not always all reference frames (last, golden, altref) are provided.
This is indicated with the flags ref_flags.bits.no_ref_last, ref_flags.bits.no_ref_gf
and ref_flags.bits.no_ref_arf in VAEncPictureParameterBufferVP8.
While checking picture parameters, make sure that assert errors concerning invalid
reference frame only occure if no_ref_X is not set.