fbdev: omapfb: delete check_required_callbacks()

Commit 561eb9d0 ("fbdev: omap/lcd: Make callbacks optional") made
panel callbacks optional but forgot to update check_required_callbacks().
As a result many (all?) OMAP systems using omapfb will crash at boot.
Fix by deleting the whole function.

Fixes: 561eb9d0 ("fbdev: omap/lcd: Make callbacks optional")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>