drm: atmel-hlcdc: remove leftovers from atomic mode setting migration

The ->dpms field is no longer used and can be removed.
The same goes for the dummy ->mode_fixup() implementation which always
returns true.

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>