drm/i915: simplify ppgtt setup

We don't need the pt_addr for the !dmar case, so drop the else and
move the if (dmar) condition out of the loop.

v2: Fixup whitespace damage noticed by Chris Wilson.

v3: Collapse the two identical if blocks. Chris Wilson makes me look
like a moron right now ...

Noticed-by: Konstantin Belousov <kostikbel@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wislon.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>