media: vimc: allocate vimc_device dynamically

In future patch, the release of the device will move
to the release callback of v4l2_device. Therefore the
device will be released only when the last fh will be
closed. Dynamic allocation will then be needed since
when the device is unbounded and then bounded again,
it might be that the probe callback will run before
the release of the last device is finished. In that
case both operations will run on the same memory
concurrently and cause memory corruption.
This patch also removes the pdev field of
vimc_device since it is not needed anymore.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>