fbdev: omap/lcd: Make callbacks optional

Most OMAP FB LCD drivers don't have anything to do in their panel
callbacks. This leads to a large set of empty boilerplate functions in the
panel drivers. Make those callbacks optional by checking if they are set
before calling them. This allows those boilerplate functions to be removed.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>