da8xx-fb: do not turn ON LCD backlight unless LCDC is enabled

LCD blink is observed during suspend/resume and blank/unblank
operations as backlight is ON during LCDC disable and enable.
So make sure to turn OFF backlight before disabling and turn
it ON after enabling.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>