Use correct luma/chroma modes in get_filt_type()

BD-rate change (4 key frames):

           PSNR    SSIM
lowres:  -0.005  +0.022
midres:  -0.016  -0.021
hdres:   -0.008  -0.015

BUG=aomedia:1312

Change-Id: Ia9a22718d330fc4d3d932dbac0723c0522f0c4d1