ALSA: hda/hdmi - fix vgaswitcheroo detection for AMD

Only enable the vga_switcheroo logic on systems with the
ATPX ACPI method.  This logic is not needed for asics
that are not part of a PX (PowerXpress)/HG (Hybrid Graphics)
platform.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/r/20191122214353.582899-2-alexander.deucher@amd.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>