agx: Model both sources of sample_mask

We need to control both sources to implement multisampling properly. The
semantic is something like:

   foreach sample in the first mask {
      if correspond bit in second bit set {
         make sample live
      } else {
         make sample dead
      }
   }

But I'm reticent to document more formally until the details are really
understood and properly tested.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23480>

src/asahi/compiler/agx_opcodes.py

@@ -292,7 +292,7 @@ op("wait", (0x38, 0xFF, 2, _), dests = 0,
 
 op("get_sr", (0x72, 0x7F | L, 4, _), dests = 1, imms = [SR])
 
-op("sample_mask", (0x7fc1, 0xffff, 6, _), dests = 0, srcs = 1, can_eliminate = False)
+op("sample_mask", (0x7fc1, 0xffff, 6, _), dests = 0, srcs = 2, can_eliminate = False)
 
 # Sources: sample mask, combined depth/stencil
 op("zs_emit", (0x41, 0xFF | L, 4, _), dests = 0, srcs = 2,