backlight: convert platform_lcd to dev_pm_ops

Instead of using legacy suspend/resume methods, using newer dev_pm_ops
structure allows better control over power management.

Signed-off-by: Jingoo Han <jg1.han@samsung.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>