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>