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

dm_dcn_crtc_high_irq() is only used when CONFIG_DRM_AMD_DC_DCN is set.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>