It seems that accessing registers during disconnection often leads to
the GPU pipe error.  The original driver had a similar check in the
past, but it was lost through refactoring.  Now put a connection check
in the register access functions.

One exception is the irq handler: it still needs to access the raw
register even while disconnected, because it has to read and write to
ACK the irq mask.  Although the irq shouldn't be raised while
disconnected (the stream should have been disabled), let's make it
safer for now.
Signed-off-by: Takashi Iwai <tiwai@suse.de>