drivers/video/backlight/lp855x_bl.c: remove unnecessary mutex code

The mutex for accessing lp855x registers is used in case of the user-space
interaction.  When the brightness is changed via sysfs, the mutex is
required.  But the backlight class device already provides it.  Thus, the
lp855x mutex is unnecessary.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>