drivers/video/backlight: rename da903x to da903x_bl

Currently both da903x backlight and voltage reulator drivers have the
same name. Rename the backlight driver to allow use of both drivers as
modules.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Eric Miao <eric.miao@marvell.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>