V4L/DVB: v4l2-common: add ability to use v4l2_ctrl_query_fill for chroma gain

Add the ability to use the v4l2_ctrl_query_fill() function for the newly
introduced chroma gain control.

Also, make use of the centralized function by the one caller.

This work was sponsored by EyeMagnet Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>