media: allegro: add helper to get entropy mode

The driver only supports CAVLC for H.264, but HEVC only uses CABAC. As
the driver has to explicitly tell the MCU to use CABAC for HEVC, add a
helper function to get the entropy mode.

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+huawei@kernel.org>