drm/i915: Clear unused fields of mode for framebuffer creation

With the stricter checks introduced in
commit ac911edae5960d7dccd9883f5fa5d25b591520de
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Wed Oct 31 17:50:19 2012 +0200

    drm/i915: Check the framebuffer offset

(and friends), it became especially prudent to make sure that the
additional fields inside the mode were cleared before attempting to
create a framebuffer. In particular, the fb created for load detection
failed to do so and hence failed.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>