modetest: include into the build when libkms is not selected.

With commit d7c0a08b(modetest: Allocate dumb buffers with the correct
bpp) we moved away from the libkms dependency. As such we are safe with
including the Makefile/subdir, even as we opt out of building the
library.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>