ALSA: hda - Increment default stream numbers for AMD HDMI controllers

It turned out that some AMD HDMI controllers still don't provide
proper values in GCAP register (all zero), and the driver assigns only
one stream in that case, although the connected codec chip supports
more than one stream.

In this patch, the default max number of streams for AMD HDMI
controllers is increased to 8, which  should suffice for most use
cases.  The overhead by this increase is more azx_dev struct and BDL
allocations, so it's negligible.  Of course, if the controller
provides a proper GCAP register, the register value would be used.

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