media: allegro: use new v4l2_m2m_ioctl_try_encoder_cmd funcs

As the try_encoder_cmd is identical for many drivers, there are now
helpers for this function in the mem2mem core. Use the helper in
allegro.

This fixes the v4l2-compliance test regarding V4L2_ENC_CMD_STOP, because
the allegro-specific function rejected invalid flags.

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+samsung@kernel.org>