nouveau: report byte usage in VRAM usage.

commit f7916c47 upstream.

Turns out usage is always in bytes not shifted.

Fixes: 72fa02fd ("nouveau: add an ioctl to report vram usage")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>