sh: Get the PGD right in oops case with 64-bit PTEs.

Previously this was using a static pgd shift in the reporting
code, simply flip this to PGDIR_SHIFT which does the right
thing depending on varying PTE magnitudes on the SH-X2 MMU.

While we're at it, and since it's been recently added, use
get_TTB() for fetching the TTB, rather than the open coded
instructions.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>