drivers: display: dummy: Enable config if `dummy-dc` device available

Enabling the configuration if there is a valid `dummy-dc`
node on the device tree.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>

drivers/display/Kconfig.dummy

@@ -5,5 +5,7 @@
 
 config DUMMY_DISPLAY
 	bool "Dummy display driver"
+	default y
+	depends on DT_HAS_ZEPHYR_DUMMY_DC_ENABLED
 	help
 	  Enable dummy display driver compliant with display driver API.