The cfl route needs the reconstructed luma pixels in the chroma
RD search. Replace an extra route of txfm_rd_in_plane with the
use of av1_encode_intra_block_plane to directly write out the
needed luma pixels in the key frame rate-distortion optimization
process.

The compression stats remain identical. The key frame encoding
speed is on average 3% faster for a few tested 1080p clips.

Change-Id: I65760b657a6d4b45bed9c4f1eca0cff1a2c274a1