HID: i2c-hid: Make elan touch controllers power on after panel is enabled

Introduce a new HID quirk to indicate that this device has to be enabled
after the panel's backlight is enabled, and update the driver data for
the elan devices to enable this quirk. This cannot be a I2C HID quirk
because the kernel needs to acknowledge this before powering up the
device and read the VID/PID. When this quirk is enabled, register
.panel_enabled()/.panel_disabling() instead for the panel follower.

Also rename the *panel_prepare* functions into *panel_follower* because
they could be called in other situations now.

Fixes: bd3cba00 ("HID: i2c-hid: elan: Add support for Elan eKTH6915 i2c-hid touchscreens")
Fixes: d06651be ("HID: i2c-hid: elan: Add elan-ekth6a12nay timing")
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Acked-by: Jiri Kosina <jkosina@suse.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250818115015.2909525-2-treapking@chromium.org