media: imx: imx-mipi-csis: Simplify runtime PM implementation

The runtime PM resume handler is guaranteed to be called on a suspended
device, and the suspend handler on a resumed device. The implementation
can thus be simplified.

While at it, rename the mipi_csis_device state field to powered, as the
now state contains a single flag only.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>