In ast2500, engine will stop occasionally for 1360x768.

This is a bug which has been addressed, but the workaround is specific
for 1680 only. Here we make it more complete.
Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>