avcodec/ac3enc: Simplify AC-3 bit counting

avcodec/ac3enc: Simplify AC-3 bit counting

When encoding E-AC-3, whether coupling is on or not determines whether
an additional frame based coupling exponent strategy element frmcplexpstr
(of size five bits) is present in the bitstream. So just add five to the
number of bits when counting them instead of adding 5*s->cpl_on (the
latter field is currently only 0 or 1, so it doesn't make a difference).

Furthermore, move some parts of the bit allocation that doesn't change
per-frame to count_frame_bits_fixed() (which is only run once during
init).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>