drm/radeon: fix hw contexts for SUMO2 asics 

commit 50b8f5aec04ebec7dbdf2adb17220b9148c99e63 upstream.

They have 4 rather than 8.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=63599

Signed-off-by: wojciech kapuscinski <wojtask9@wp.pl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>