drm/atmel-hlcdc: Remove unnecessary NULL check

drm_fbdev_cma_hotplug_event() already checks for NULL pointers before
dereferencing, so callers don't need to do that.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170329144401.1804-11-thierry.reding@gmail.com