powerpc: Dynamically calculate the dabrx based on kernel/user/hypervisor

Currently we mark the DABRX to interrupt on all matches
(hypervisor/kernel/user and then filter in software.  We can be a lot
smarter now that we can set the DABRX dynamically.

This sets the DABRX based on the flags passed by the user.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>