avcodec/h264: reenable ER except with slice threads by default

ER with slice threads is buggy and since the merge of the libav cleanup broken
as the ER context which is supposed to be per frame has been placed in
the slice context, so there are multiple per frame which does not work as is.

Theres no bug report about ER with frame threads. If someone knows of a
case where it crashes / fails without slice threads please mail me and
open a ticket on trac.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
