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>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Tested-by: Jian-Hong Pan <jhp@endlessos.org>
Link: https://lore.kernel.org/r/20211117094527.146275-3-maxime@cerno.tech