This commit only factors out freeing the common SEI parts,
not whether the fields indicating whether an SEI is present
shall be reset. H.264 and HEVC differ in this regard
(ff_h264_sei_uninit() really resets, whereas ff_hevc_reset_sei()
only uninits.) and neither actually honours the persistency
as prescribed by the relevant specs.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>