drm: atmel-hlcdc: check display mode validity in crtc->mode_fixup()

Move the adjusted display mode check into ->mode_fixup().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Nicolas Ferre <nicolas.ferre@atmel.com>