drm/i915: work around warning in i915_gem_gtt

Gcc warns that addr might be used uninitialized. It may not, but I see
why gcc gets confused.

Additionally, hiding code with side-effects inside WARN_ON() argument
seems uncool, so I moved it outside.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
[danvet: Add obligatory /* shuts up gcc */ comment.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>