drm/amdgpu/display: fix build with CONFIG_DRM_AMD_DC_DCN disabled

dc_allow_idle_optimizations() needs to be protected by
CONFIG_DRM_AMD_DC_DCN.

Fixes: 71338cb4 ("drm/amd/display: enable idle optimizations for linux (MALL stutter)")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>