drm: atmel-hlcdc: avoid initializing cfg with zero

Remove cfg initialization with zero and read state with
drm_crtc_state_to_atmel_hlcdc_crtc_state() so that cfg to be initialized
with state's output_mode.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1556195748-11106-3-git-send-email-claudiu.beznea@microchip.com