Unlike HSW and newer, BYT/CHT devices have no fine control of audio
power domain in i915 side.  Since there is little gain by runtime PM
on BYT/CHT, so far, this commit removes the pm_runtime_enable() call
as default.

User who still wants the runtime PM may adjust the corresponding
sysfs files (power/control and power/autosuspend_delay_ms)
appropriately, of course.

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