powerpc/kprobes: Factor out code to emulate instruction into a helper

Factor out code to emulate instruction into a try_to_emulate()
helper function. This makes no functional changes.

Acked-by: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>