media: allegro: skip filler data if possible

The driver instructs the firmware to leave some space space in front of
the coded frame data for SPS/PPS data. If the driver receives an IDR, it
writes the SPS/PPS into that free space and fills the rest with filler
data. However, if there is no additional data, the driver can use the
plane offset to skip this space instead of adding filler data.

As the size of the SPS/PPS is only available after writing it, keep the
filler data between the SPS/PPS and the coded frame data.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>