dtoverlays: Drop i2c baudrate for Pi 7inch DSI screen to 50kHz

Whilst investigations continue as to why the 7" DSI screen doesn't
work on Pi3 with KMS, drop the I2C baudrate to 50kHz as that seems
to be a reliable workaround.
This will be reverted once the full issue is understood.

https://github.com/raspberrypi/linux/issues/4686



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>