Currently the driver handles some reset procedure at the trigger STOP
and the underrun functions, where both are executed in the interrupt
context.  Especially the underrun function has a sync-loop to clear
the UNDERRUN status bit, and this is supposed to be one of plausible
causes of GPU hangup.

Since the job to be done in the interrupt handler should be minimum,
we move the reset function out of trigger and underrun, and push it
into the prepare (and hw_free) callbacks instead.  Here a new flag,
need_reset, is introduced to indicate the requirement of the reset
procedure.  This is for avoiding the multiple resets when PCM prepare
is called sequentially.

Also in the UNDERRUN bit-clear sync loop, take a longer pause to be in
the safer side.  Taking a longer delay is no longer a problem now
because we're running in the normal context.
Signed-off-by: Takashi Iwai <tiwai@suse.de>