Simplify constant q mode qp selection

Decouple the constant q mode qp selection from vbr/cbr/cq modes.
Skip vp9_frame_type_qdelta() adjustment for non-ARF inter frames,
instead keep using the cq-level. It improves the compresson
performance:

         avg PSNR       overall PSNR     SSIM
lowres   -0.17%         -0.20%           -0.1%
midres   -0.21%         -0.24%           -0.08%
hdres    -0.15%         -0.19%           -0.04%

Change-Id: I52fd5f8edbd3fdcbeda31ee3a6d6eb016091a7e3
1 file changed