video: of_display_timing.h: Declare 'display_timing'

Commit ffa3fd21 ("videomode: implement public of_get_display_timing()") causes
the following build warning:

include/video/of_display_timing.h:18:10: warning: 'struct display_timing' declared inside parameter list [enabled by default]
include/video/of_display_timing.h:18:10: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Declare 'display_timing' to avoid the build warning.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>