backlight: Add missing MODULE_DESCRIPTION() macros

With ARCH=x86, make allmodconfig && make W=1 C=1 reports:

 WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/video/backlight/platform_lcd.o
 WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/video/backlight/rt4831-backlight.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240612-md-drivers-video-backlight-v1-1-f4ca1beb36cc@quicinc.com


Signed-off-by: Lee Jones <lee@kernel.org>