media: v4l2-core: Initialize h264 scaling matrix

In the final H264 API, it is not required to set scaling matrix if
they are not present in the bitstream. A flag was added in order to let
the driver know. The downside is that it leaves the default control
value to 0, which isn't valid. As per the spec (see formulas 7-8/7-9),
when the scaling matrix are absent from the bitstream, flat values
of 16 should be used. This improves this control semantic in a way
that the control value are always valid. Drivers can then use
the scaling_matrix control values without having to check its presence.
Same method was employed for MPEG2_QUANTISATION.

This fixes issues with MTK VCODEC H264 decoder when using GStreamer.
GStreamer does not set this control if its not present in the bitstream.
As MTK VDCODEC was using the initialized to 0 values, the frames ended
up completely gray.

Fixes: 54889c51 ("media: uapi: h264: Rename and clarify PPS_FLAG_SCALING_MATRIX_PRESENT")
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>