drm/amdgpu: use read-modify-write mode for gfx v9_4_3 SQ setting

Instead of using direct update, avoid touching unrelated fields.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>