radeonsi: use hardware scissors correctly

radeonsi: use hardware scissors correctly

Use the WINDOW and VPORT scissors for the framebuffer and scissor test,
respectively. The other two scissors are disabled (they cover the max fb size).

We actually have 16 VPORT scissors, which will map well to ARB_viewport_array.

Also, we don't need to write SC_WINDOW_OFFSET with this commit, because it's
disabled everywhere.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>