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: NTomi Valkeinen <tomi.valkeinen@ti.com>