drm/amd/display/dc/gpio/diagnostics/hw_factory_diag: Fix struct declared inside parameter list error

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:43: warning: ‘struct hw_factory’ declared inside parameter list will not be visible outside of this definition or declaration
drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:6: note: previous declaration of ‘dal_hw_factory_diag_fpga_init’ was here
drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:43: warning: ‘struct hw_factory’ declared inside parameter list will not be visible outside of this definition or declaration
drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:6: note: previous declaration of ‘dal_hw_factory_diag_fpga_init’ was here
drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:43: warning: ‘struct hw_factory’ declared inside parameter list will not be visible outside of this definition or declaration
drivers/gpu/drm/amd/amdgpu/../display/dc/gpio/diagnostics/hw_factory_diag.h:30:6: note: previous declaration of ‘dal_hw_factory_diag_fpga_init’ was here

Cc: Harry Wentland <[e-mail]>
Cc: Leo Li <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/gpio/diagnostics/hw_factory_diag.h

···
 #ifndef __DAL_HW_FACTORY_DIAG_FPGA_H__
 #define __DAL_HW_FACTORY_DIAG_FPGA_H__
 
+struct hw_factory;
+
 /* Initialize HW factory function pointers and pin info */
 void dal_hw_factory_diag_fpga_init(struct hw_factory *factory);