Register 0x13 seems to be a sort of image control, maybe gamma, white
level or black level. Lower values produce better images, while higher
values increases the contrast and shifts colors to green. 0xff produces
a black image. This register is not Silvercrest-specific, so its code
should be moved to a better place.

If this register is left alone, a random value can be found at the
register, producing weird results.

While here, let's remove register 0x0d, as it had no noticed effect at
the image.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>