drm/amd/display: Add some forward declarations

[WHAT]
Add DML2 pipe and config struct forward declaration as a
preparation for DML2.
Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml2/dml2_dc_resource_mgmt.h

@@ -30,6 +30,8 @@
 #include "dml2_dc_types.h"
 
 struct dml2_context;
+struct dml2_dml_to_dc_pipe_mapping;
+struct dml_display_cfg_st;
 
 /*
  * dml2_map_dc_pipes - Creates a pipe linkage in dc_state based on current display config.