drm/i2c/ch7006: Drop build time dependency to nouveau.

This partially reverts e4b41066, as this driver is intended to be
useful with any KMS driver for suitable hardware. The missing build
dependency that commit workarounded was DRM_KMS_HELPER.
Signed-off-by: Francisco Jerez <currojerez@riseup.net>

drivers/gpu/drm/nouveau/Kconfig

@@ -30,12 +30,11 @@ config DRM_NOUVEAU_DEBUG
 	  via debugfs.
 
 menu "I2C encoder or helper chips"
-     depends on DRM && I2C
+     depends on DRM && DRM_KMS_HELPER && I2C
 
 config DRM_I2C_CH7006
 	tristate "Chrontel ch7006 TV encoder"
-	depends on DRM_NOUVEAU
-	default m
+	default m if DRM_NOUVEAU
 	help
 	  Support for Chrontel ch7006 and similar TV encoders, found
 	  on some nVidia video cards.