radv: Make FragCoord a sysval

load_fragcoord is already handled in common code for radeonsi, so we
don't need to do anything to handle it. However, there were some passes
creating NIR with the varying, so we switch them over to the sysval. In
the case of nir_lower_input_attachments which is used by both radv and
anv, we add handling for both until intel switches to using a sysval.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/compiler/nir/nir.h

@@ -3494,7 +3494,7 @@ bool nir_lower_non_uniform_access(nir_shader *shader,
 
 bool nir_lower_idiv(nir_shader *shader);
 
-bool nir_lower_input_attachments(nir_shader *shader);
+bool nir_lower_input_attachments(nir_shader *shader, bool use_fragcoord_sysval);
 
 bool nir_lower_clip_vs(nir_shader *shader, unsigned ucp_enables, bool use_vars);
 bool nir_lower_clip_fs(nir_shader *shader, unsigned ucp_enables);