drm/amd/display: remove unnecessary else by CONFIG_DRM_AMD_DC_DCN

[Why & How]
CONFIG_DRM_AMD_DC_DCN is used by pass the compilation failures, but DC
code should be OS-agnostic.

This patch fixes it by removing unnecessasry CONFIG_DRM_AMD_DC_DCN
guards for #if-#else clause.
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -2315,14 +2315,10 @@ enum dc_status resource_map_pool_resources(
 
 	/* Add ABM to the resource if on EDP */
 	if (pipe_ctx->stream && dc_is_embedded_signal(pipe_ctx->stream->signal)) {
-#if defined(CONFIG_DRM_AMD_DC_DCN)
 		if (pool->abm)
 			pipe_ctx->stream_res.abm = pool->abm;
 		else
 			pipe_ctx->stream_res.abm = pool->multiple_abms[pipe_ctx->stream_res.tg->inst];
-#else
-		pipe_ctx->stream_res.abm = pool->abm;
-#endif
 	}
 
 	for (i = 0; i < context->stream_count; i++)