media: exynos4-is: Request syscon only if ISP writeback is present

On FIMC variants which don't have writeback channel, there is no need to
access system registers. This patch makes the driver request sysreg
regmap conditionally depending on whether writeback is supported.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>