drm/amdgpu: remove structurally dead code

This code cannot be reached: return "UNKNOWN";.
Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h

@@ -554,8 +554,6 @@ static inline const char *amdgpu_gfx_compute_mode_desc(int mode)
 	default:
 		return "UNKNOWN";
 	}
-
-	return "UNKNOWN";
 }
 
 #endif