ARM: dts: bcm2711: Tidy the HDMI I2C aliases

The bcm2711 vc3-kms-v3d overlay enables the I2C instances used for
EDID data. Give these distinct I2C interface numbers (20 & 21) to
clearly separate them from other regular I2C blocks (1, 3-6) and the
mux on I2C0 (10+).

The 2711 DTS tree no longer includes i2c2, so the explicit deletion can
be removed.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>