drm/vc4: Fix handling of a pm_runtime_get_sync() success case.

If the device was already up, a 1 is returned instead of 0.  We were
erroring out, leading the 3D driver to sometimes fail at screen
initialization (generally with ENOENT returned to it).

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: af713795 ("drm/vc4: Add a getparam ioctl for getting the V3D identity regs.")