Indeed, regular P frames are not supported yet. Please refer to HEVC encoder section in release notes of MSS 2017 R2:
Encoder cannot encode regular P-frames. Low-delay B-frames are coded instead. All parameters targeted to P-frames (e.g. GopRefDist, -QPP) are applied to these low-delay B frames (new). Similarly, if FrameType = MFX_FRAMETYPE_P is passed in encoded order, it's coded as low-delay B-frame.