media: add panel register access functions

These functions provide write access to the internal LCD panel registers
which also control the sensor. They can be used to disable the
preprocessor, set the illumination brightness, and adjust gain/contrast
(which are stored together in one register internally called "vsvideo").

Signed-off-by: Florian Echtler <floe@butterbrot.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>