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: 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 Shish <Jude.Shih@amd.com>