drm/bridge/sii902x: Add missing dependency on I2C_MUX

kbuild test robot reports:

>> ERROR: "i2c_mux_add_adapter" [drivers/gpu/drm/bridge/sii902x.ko] undefined!
>> ERROR: "i2c_mux_alloc" [drivers/gpu/drm/bridge/sii902x.ko] undefined!
>> ERROR: "i2c_mux_del_adapters" [drivers/gpu/drm/bridge/sii902x.ko] undefined!

Quite obviously the driver depends on I2C_MUX, but adding a "depends on"
introduces a recursive dependency, therefore this patch selects I2C_MUX
instead.

Fixes: 21d8084 ("drm/bridge/sii902x: Fix EDID readback")
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Link: https://lists.01.org/pipermail/kbuild-all/2018-November/054924.html
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1542633978-22064-1-git-send-email-fabrizio.castro@bp.renesas.com

Author: Fabrizio Castro

drivers/gpu/drm/bridge/Kconfig

@@ -95,6 +95,7 @@ config DRM_SII902X
 	depends on OF
 	select DRM_KMS_HELPER
 	select REGMAP_I2C
+	select I2C_MUX
 	---help---
 	  Silicon Image sii902x bridge chip driver.