da8xx-fb: enable LCDC if FB is unblanked

It is expected that LCDC to continue to be disabled after
resume if it is blanked before suspend. This is also true
for DVFS. But it is observed that LCDC being enabled after
suspend/resume cycle or DVFS.

Correcting it by having check for FB_BLANK_UNBLANK before
enabling.

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