drm: Use static attribute groups for managing connector sysfs entries

Instead of manual calls of device_create_file() and
device_remove_file(), assign the static attribute groups to the device
with device_create_with_groups().  The conditionally built sysfs
entries are handled via is_visible callback.

This simplifies the code and also avoids the possible races.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>