drm/vc4: kms: Fix return code check

commit f9277679 upstream.

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


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>