Fix RD multiplier bug impacting AQ1.

Fix RD multiplier bug impacting AQ1.

Change to the default RD multiplier computation in set_segment_rdmult()

The default here is wrong as for modes like AQ 1 setting the rdmult based on the
segment ID for bsize will tend to result in the RD loop favoring partition sizes where
the resulting segment assignment has the lowest Q, as these partition sizes will be
then evaluated with a lower value of rdmult. For a valid rd comparison between
partition sizes within a single SB64 we need to use the same value of rdmult.

This change fixes an observed issue with AQ 1 where almost all the blocks were being
assigned to segment 0.

Change-Id: Ibf87e8ca60bca45b8fee866ac6fd53feae11dab4