drm/gem-fb-helper: Use debug message on gem lookup failure

GEM lookup failure can easily be triggered by userspace so make
it a debug message, not an error message.

Also remove unnecessary inner parentheses and fix alphabetical
struct declaration order.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505147865-18194-2-git-send-email-noralf@tronnes.org