drm/amdgpu/gfx8: fix CP jump table size

Align to the jump table offset. Fixes hangs on some
systems with GFX PG enabled.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>