media: cedrus: H265: Fix neighbour info buffer size

Neighbour info buffer size needs to be 794 kiB in H6. This is actually
already indirectly mentioned in the comment, but smaller size is used
nevertheless.

Increase buffer size to cover H6 needs. Since increase is not that big
in absolute numbers, it doesn't make sense to complicate logic for older
generations.

Bug was discovered using iommu, which reported access error when trying
to play H265 video.

Fixes: 86caab29 ("media: cedrus: Add HEVC/H.265 decoding support")
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>