media: imx: imx-mipi-csis: Protect mipi_csis_dump_regs()

The mipi_csis_dump_regs() function accesses the interface registers
in order to printout their values for debug purposes.

As the function access the registers, it requires the interface to be
powered up. Currently this is only enforced in one of the function's
callers (mipi_csis_log_status)() but not when the function is called by
the debugfs attribute handler.

Make sure to access registers only if the interface is powered up and
remove the same check from the caller.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>