drm/i915: Use phys_addr_t for the address of stolen memory

Though we know the hw is limited to keeping stolen memory inside the
first 4GiB, it is clearer to the reader that we are handling physical
address if we use phys_addr_t to refer to the base of stolen memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106152013.24684-2-chris@chris-wilson.co.uk