ac/radeonsi: add lookup_interp_param and load_sample_position to the abi

This will enable the interpolateAt builtins to work on the radeonsi
nir backend.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/amd/common/ac_shader_abi.h

@@ -147,6 +147,13 @@ struct ac_shader_abi {
 					  enum ac_descriptor_type desc_type,
 					  bool image, bool write);
 
+	LLVMValueRef (*lookup_interp_param)(struct ac_shader_abi *abi,
+					    enum glsl_interp_mode interp,
+					    unsigned location);
+
+	LLVMValueRef (*load_sample_position)(struct ac_shader_abi *abi,
+					     LLVMValueRef sample_id);
+
 	/* Whether to clamp the shadow reference value to [0,1]on VI. Radeonsi currently
 	 * uses it due to promoting D16 to D32, but radv needs it off. */
 	bool clamp_shadow_reference;