drm/vc4: kms: Fix return code check

The HVS global state functions return an error pointer, but in most
cases we check if it's NULL, possibly resulting in an invalid pointer
dereference.

Fixes: 9ec03d7f ("drm/vc4: kms: Wait on previous FIFO users before a commit")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>