media: v4l2-ctrls: split up into four source files

The v4l2-ctrls.c source has become much too big, so split it up
into four separate parts:

v4l2-ctrls-core.c: contains the core framework code
v4l2-ctrls-api.c: contains the uAPI interface to the framework
v4l2-ctrls-defs.c: contains the control definitions
v4l2-ctrls-request.c: contains the Request API helpers

And it adds a new v4l2-ctrls-priv.h.

No code was changed, but a number of checkpatch.pl warnings were
fixed (alignment, f == NULL -> !f, long comment block coding style,
unsigned -> unsigned int).

The copyright statements were updated as well since they were
quite out of date.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>