powerpc: Fix program check handling when lockdep is enabled

In commit 54321242 ("Disable interrupts early in Program Check"), we
switched from enabling to disabling interrupts in program_check_common.

Whereas ENABLE_INTS leaves r3 untouched, if lockdep is enabled DISABLE_INTS
calls into lockdep code and will clobber r3. That means we pass a bogus
struct pt_regs* into program_check_exception() and all hell breaks loose.

So load our regs pointer into r3 after we call DISABLE_INTS.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>