nvc0: set rsvd_kick correctly

This prevents trampling beyond the end of the command stream during flushes.

NOTE: This is a candidate for the stable branches.

Reported-by: Christoph Bumiller <christoph.bumiller@speed.at>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
(cherry picked from commit e72cc26518d8a60093ac497bf680c1f4ba614fa9)