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>

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

@@ -5490,10 +5490,12 @@ static inline int dm_set_vblank(struct drm_crtc *crtc, bool enable)
 	else
 		dm->active_vblank_irq_count--;
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 	dc_allow_idle_optimizations(
 		adev->dm.dc, dm->active_vblank_irq_count == 0 ? true : false);
 
 	DRM_DEBUG_DRIVER("Allow idle optimizations (MALL): %d\n", dm->active_vblank_irq_count == 0);
+#endif
 
 	mutex_unlock(&dm->dc_lock);