asahi: eliminate troublesome empty uniforms

these don't do what you expect and are wasteful.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29179>

src/asahi/lib/agx_usc.h

@@ -76,6 +76,7 @@ agx_usc_uniform(struct agx_usc_builder *b, unsigned start_halfs,
 {
    assert((start_halfs + size_halfs) <= (1 << 9) && "uniform file overflow");
    assert(size_halfs <= 64 && "caller's responsibility to split");
+   assert(size_halfs > 0 && "no empty uniforms");
 
    if (start_halfs & BITFIELD_BIT(8)) {
       agx_usc_pack(b, UNIFORM_HIGH, cfg) {

src/gallium/drivers/asahi/agx_nir_lower_sysvals.c

@@ -375,19 +375,22 @@ lay_out_uniforms(struct agx_compiled_shader *shader, struct state *state)
          DIV_ROUND_UP(BITSET_LAST_BIT(shader->attrib_components_read), 4);
 
       struct agx_draw_uniforms *u = NULL;
-      shader->push[shader->push_range_count++] = (struct agx_push_range){
-         .uniform = 0,
-         .table = AGX_SYSVAL_TABLE_ROOT,
-         .offset = (uintptr_t)&u->attrib_base,
-         .length = 4 * count,
-      };
 
-      shader->push[shader->push_range_count++] = (struct agx_push_range){
-         .uniform = 4 * count,
-         .table = AGX_SYSVAL_TABLE_ROOT,
-         .offset = (uintptr_t)&u->attrib_clamp,
-         .length = 2 * count,
-      };
+      if (count) {
+         shader->push[shader->push_range_count++] = (struct agx_push_range){
+            .uniform = 0,
+            .table = AGX_SYSVAL_TABLE_ROOT,
+            .offset = (uintptr_t)&u->attrib_base,
+            .length = 4 * count,
+         };
+
+         shader->push[shader->push_range_count++] = (struct agx_push_range){
+            .uniform = 4 * count,
+            .table = AGX_SYSVAL_TABLE_ROOT,
+            .offset = (uintptr_t)&u->attrib_clamp,
+            .length = 2 * count,
+         };
+      }
 
       shader->push[shader->push_range_count++] = (struct agx_push_range){
          .uniform = 6 * count,