It's been reported that, when neither nouveau nor Nvidia graphics
driver is used, the screen starts flickering.  And, after comparing
between the working case (stable 4.4.x) and the broken case, it turned
out that the problem comes from the audio component binding.  The
Nvidia and AMD audio binding code clears the bus->keep_power flag
whenever snd_hdac_acomp_init() succeeds.  But this doesn't mean that
the component is actually bound, but it merely indicates that it's
ready for binding.  So, when both nouveau and Nvidia are blacklisted
or not ready, the driver keeps running without the audio component but
also with bus->keep_power = false.  This made the driver runtime PM
kicked in and powering down when unused, which results in flickering
in the graphics side, as it seems.

For fixing the bug, this patch moves the bus->keep_power flag change
into generic_acomp_notifier_set() that is the function called from the
master_bind callback of component ops; i.e. it's guaranteed that the
binding succeeded.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=208609
Fixes: 5a858e79 ("ALSA: hda - Disable audio component for legacy Nvidia HDMI codecs")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200728082033.23933-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>