vaapi_encode: Use gop_size consistently in RC parameters

The non-H.26[45] codecs already use this form.  Since we don't
currently generate I frames for codecs which support them separately
to IDR, the p_per_i variable is set to infinity by default so that it
doesn't interfere with any other calculation.  (All the code for I
frames still exists, and it works for H.264 if set manually.)