drm/radeon: fix hw contexts for SUMO2 asics

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>
Cc: stable@vger.kernel.org