media: imx: imx7_mipi_csis: Fully initialize MIPI_CSIS_DPHYCTRL register

When setting the CSIS parameters, write the MIPI_CSIS_DPHYCTRL register
fully instead of modifying selected fields, as the register doesn't
contain any reserved fields that need to be preserved. This simplifies
initialization slightly, and ensures that the register value doesn't
depend on its previous state (before a warm reboot for instance).

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>