DRM: armada: fix missing DRM_KMS_FB_HELPER select

Commit 92b6f89f6b8f (drm: Add separate Kconfig option for fbdev helpers)
happened in parallel with the inclusion of Armada DRM into mainline,
and so missed this update.  Add the necessary select statement to avoid
build errors.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

drivers/gpu/drm/armada/Kconfig

@@ -5,6 +5,7 @@ config DRM_ARMADA
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
 	select DRM_KMS_HELPER
+	select DRM_KMS_FB_HELPER
 	help
 	  Support the "LCD" controllers found on the Marvell Armada 510
 	  devices.  There are two controllers on the device, each controller