drm/vc4: Fix clock source for VEC PixelValve on BCM2711

On the BCM2711 (Raspberry Pi 4), the VEC is actually connected to
output 2 of pixelvalve3.

NOTE: This contradicts the Broadcom docs, but has been empirically
tested and confirmed by Raspberry Pi firmware devs.

Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>