drm/amdgpu/display: remove an old DCN3 guard

The DCN3 guards were dropped a while ago, this one must have
snuck in in a merge or something.
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3720,10 +3720,8 @@ static int amdgpu_dm_initialize_drm_device(struct amdgpu_device *adev)
 
 	/* Use Outbox interrupt */
 	switch (adev->asic_type) {
-#if defined(CONFIG_DRM_AMD_DC_DCN3_0)
 	case CHIP_SIENNA_CICHLID:
 	case CHIP_NAVY_FLOUNDER:
-#endif
 	case CHIP_RENOIR:
 		if (register_outbox_irq_handlers(dm->adev)) {
 			DRM_ERROR("DM: Failed to initialize IRQ\n");