broadcom/compiler: Set up passthrough Z when doing FS discards.

In order to keep early-Z from writing early in a discard shader, you need
to set the "modifies Z" bit in the shader state (which the new
prog_data.discards will indicate).  Then, in the shader we do a TLB write
to make Z passthrough happen (the QPU result is ignored, so we use a NULL
source).

src/broadcom/compiler/v3d_compiler.h

@@ -579,6 +579,7 @@ struct v3d_fs_prog_data {
         BITSET_WORD flat_shade_flags[BITSET_WORDS(V3D_MAX_FS_INPUTS)];
 
         bool writes_z;
+        bool discard;
 };
 
 /* Special nir_load_input intrinsic index for loading the current TLB