Fix incorrect macroblock counts in twopass rate control

The previous calculation of macroblock count (w*h)/256
is not correct when the width/height are not multiples of
16. Use the precalculated macroblock count from
cpi->common instead. This manifested itself as a divide
by zero when the number of pixels was less than 256.
num_mbs updated in estimate_max_q, estimate_q,
 estimate_kf_group_q, and estimate_cq

Change-Id: I92ff98587864c801b1ee5485cfead964673a9973