libkms: add dumb support

this uses the drm cap interface to check if the dumb ioctl
is supported.
diff --git a/libkms/Makefile.am b/libkms/Makefile.am
index 5c2e63f..fa379a4 100644
--- a/libkms/Makefile.am
+++ b/libkms/Makefile.am
@@ -16,6 +16,7 @@
 	internal.h \
 	linux.c \
 	intel.c \
+	dumb.c \
 	api.c
 
 if HAVE_VMWGFX