media: v4l2-core: Rename array 'video_driver' to 'video_drivers'

Improves code clarity in two ways:
1. The plural name makes it more clear that it is an array.
2. The name of the array is now no longer identical to the struct type
name, so it is easier to find in the code.

Signed-off-by: Simon Que <sque@chromium.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>