media: imx: imx7-media-csi: Don't lock access to is_csi2

The is_csi2 field can't be accessed concurrently by
imx7_csi_pad_link_validate() and imx7_csi_configure(), as the latter is
called from imx7_csi_s_stream(), which is called after link validation.
Drop the lock.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>