OMAPDSS: add missing include for string.h

Both dpi.c and sdi.c use strcmp(), but do not include string.h. With
some Kconfig options string.h is included implicitly, but with some
other the compilation fails:

drivers/video/omap2/dss/dpi.c:407:5: error: implicit declaration of
function 'strcmp'

Include string.h in both dpi.c and sdi.c

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>