panfrost: Rename VARYING_DISCARD to CONSTANT

Used for data that does not exist. If used for a load, it is a
zero-components read (so you can use a 0000 or 0001 swizzle) that does
not touch memory. If used as a store, writes are simply discarded.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7352>

src/gallium/drivers/panfrost/pan_cmdstream.c

@@ -1476,7 +1476,7 @@ static void
 pan_emit_vary_only(struct mali_attribute_packed *out,
                 unsigned present, unsigned quirks)
 {
-        pan_emit_vary(out, present, 0, quirks, MALI_VARYING_DISCARD, 0);
+        pan_emit_vary(out, present, 0, quirks, MALI_CONSTANT, 0);
 }
 
 /* Special records */

src/panfrost/lib/midgard.xml

@@ -138,7 +138,7 @@
     <!--- Format 93 avaible v7+. On Midgard, acts as a RG16F + snap -->
     <value name="S8" value="93"/>
     <value name="VARYING POS" value="94"/>
-    <value name="VARYING DISCARD" value="95"/>
+    <value name="Constant" value="95"/>
     <value name="R8 SNORM" value="99"/>
     <value name="R16 SNORM" value="100"/>
     <value name="R32 SNORM" value="101"/>