drm/amdgpu/display: fix build when CONFIG_DRM_AMD_DC_DCN is not defined

Fixes:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c: In function ‘amdgpu_dm_initialize_drm_device’:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:3726:7: error: implicit declaration of function ‘register_outbox_irq_handlers’; did you mean ‘register_hpd_handlers’? [-Werror=implicit-function-declaration]
 3726 |   if (register_outbox_irq_handlers(dm->adev)) {
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |       register_hpd_handlers

Fixes: 81927e2 ("drm/amd/display: Support for DMUB AUX")
Reviewed-by: Jude Shih <shenshih@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Jude Shish <Jude.Shih@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3718,6 +3718,7 @@ static int amdgpu_dm_initialize_drm_device(struct amdgpu_device *adev)
 			goto fail;
 		}
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 	/* Use Outbox interrupt */
 	switch (adev->asic_type) {
 	case CHIP_SIENNA_CICHLID:
@@ -3731,6 +3732,7 @@ static int amdgpu_dm_initialize_drm_device(struct amdgpu_device *adev)
 	default:
 		DRM_DEBUG_KMS("Unsupported ASIC type for outbox: 0x%X\n", adev->asic_type);
 	}
+#endif
 
 	/* loops over all connectors on the board */
 	for (i = 0; i < link_cnt; i++) {