ALSA: hda/realtek - Allow skipping spec->init_amp detection

Some devices have the overrides of spec->init_amp at
HDA_FIXUP_ACT_PROBE just because alc_ssid_check() gives the
false-positive values from the SSID.

For more consistent behavior, define the logic in the following way:

- Define ALC_INIT_UNDEFINED as the default value before calling
  alc_ssid_check()
- Each fixup may set up spec->init_amp with another value at
  HDA_FIXUP_ACT_PRE_PROBE
- At detection, check whether spec->init_amp is ALC_INIT_UNDEFINED or
  not; if it's different, we skip the detection

Also, it turned out that ASUS TX300 requires the spec->init_amp
override, too; currently it ignores the GPIO bits implicitly by its
static init verb, but this will be changed in the later patchset.

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