media: renesas: vsp1: Drop custom .get_fmt() handler for histogram

media: renesas: vsp1: Drop custom .get_fmt() handler for histogram

The histogram module is the only one that has a custom .get_fmt()
handler, to handle the special case of the output format being fixed.
This can equally well be handled in the .set_fmt() handler instead.
Beside avoiding special cases and using the same .get_fmt() handler in
all modules, it ensures that the correct format is stored in the active
state for the source pad, including when .set_fmt() is called from
vsp1_entity_init_state(). Both are needed to later switch to the V4L2
subdev active state API.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi+renesas@ideasonboard.com>