drm/i915: Use drm_for_each_fb in i915_debugfs.c

Just so I have a user for this macro.

v2: Use the right macro - somehow I thought gcc should scream at me,
but list_for_each isn't really typesafe unfortunately. Spotted by
Ville.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>