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

Fixes:
At top level:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:633:13: warning: ‘dm_dmub_outbox1_low_irq’ defined but not used [-Wunused-function]
  633 | static void dm_dmub_outbox1_low_irq(void *interrupt_params)
      |             ^~~~~~~~~~~~~~~~~~~~~~~

Fixes: 81927e28 ("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 Shih <Jude.Shih@amd.com>

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

@@ -620,7 +620,6 @@ static void dm_dcn_vertical_interrupt0_high_irq(void *interrupt_params)
 	amdgpu_dm_crtc_handle_crc_window_irq(&acrtc->base);
 }
 #endif
-#endif
 
 /**
  * dm_dmub_outbox1_low_irq() - Handles Outbox interrupt
@@ -673,6 +672,7 @@ static void dm_dmub_outbox1_low_irq(void *interrupt_params)
 
 	ASSERT(count <= DMUB_TRACE_MAX_READ);
 }
+#endif
 
 static int dm_set_clockgating_state(void *handle,
 		  enum amd_clockgating_state state)