nvc0: restore viewport after blit

Based on calim's original fix in the nine branch.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: "9.2 and 9.1" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit ad4dc7723199b3052231f5b4aed1bd1ba1505025)

Conflicts:
	src/gallium/drivers/nvc0/nvc0_context.h