Update rdcost using the rd_mult in current block

Update rdcost using the rd_mult in current block

This CL is a preparation for implementing hierarchical SSIM rdmult scaling.
There is very little impact on metrics and speed:
       avg_psnr ovr_psnr  ssim
midres   0.009   0.009   0.015

perf stat -e instructions:u ./vpxenc park_joy_480p.y4m --limit=50
with this cl: 317,722,808,461
before:       317,700,108,619

Change-Id: I7b1d1482ac69f7bc87065a93223a0274bcbe8ce3