drm/i915: Simplify guard logic for setup_scratch_page()

Older gcc is complaining it can't follow the guards and thinks that
addr may be used uninitialised

In the process, we can simplify down to one loop,
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-131 (-131)
Function                                     old     new   delta
setup_scratch_page                           545     414    -131

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129102840.19901-1-chris@chris-wilson.co.uk