ALSA: hda - Split Thinkpad ACPI-related code

Both patch_realtek.c and patch_conexant.c contain the fairy same code
snippet for supporting Thinkpad ACPI LED controls.  Split them into
thinkpad_helper.c and include it from both places.  Although this
isn't the best approach from the code size POV, the probability for
coexistence of both Realtek and Conexant codecs on a single machine is
pretty low, thus it'll end up with less memory footprint than
splitting to yet another module.

Signed-off-by: Takashi Iwai <tiwai@suse.de>