Update num_mbs_left When mb_x is Reset.

When we reset the mb_x values, the num_mbs_left should also be
reset accordingly. Added the code for the same in
impeg2d_dec_pnb_mb_params. Also, there was nothing to do in p
frames when the first_mb was set to 1. Added code for setting
mb_x value and updating the num_mbs left similar to
"impeg2d_dec_pnb_mb_params" function.

Bug: 63874456
Test: run PoC through before/after ASAN build
Change-Id: I7a0bfc33e22c0e8cb93ff29198f30052f7f96546