media: cedrus: h265: Fix configuring bitstream size

bit_size field holds size of slice, not slice + header. Because of HW
quirks, driver can't program in just slice, but also preceding header.
But that means that currently used bit_size is wrong (too small).
Instead, just use size of whole buffer. There is no harm in doing this.

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