drm/i915: Make display gtt offsets u32

Using 'unsigned long' for ggtt offsets doesn't make much sense. Use
'u32' instead since we've not yet seen a >4GiB ggtt.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453316739-13296-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>