avcodec: Remove cumbersome way of checking for amount of bytes left

avcodec: Remove cumbersome way of checking for amount of bytes left

Several encoders used code like the following to check for the amount of
bytes left in a PutBitContext:
pb->buf_end - pb->buf - (put_bits_count(pb) >> 3)
Besides the fact that using the pointers directly might pose
a maintainence burden in the future this also leads to suboptimal code:
The above code reads all three pointers (buf, buf_ptr and buf_end), but
touching buf is unnecessary and switching to put_bytes_left()
automatically fixes this.

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