drm/amd/display: Return a correct error value

[why]
The function is expected to return instance of the timing generator
therefore we shouldn't be returning boolean in integer function,
and we shouldn't be returning zero so changing it to -1.
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1866,7 +1866,7 @@ static int acquire_resource_from_hw_enabled_state(
 	inst = link->link_enc->funcs->get_dig_frontend(link->link_enc);
 
 	if (inst == ENGINE_ID_UNKNOWN)
-		return false;
+		return -1;
 
 	for (i = 0; i < pool->stream_enc_count; i++) {
 		if (pool->stream_enc[i]->id == inst) {
@@ -1878,10 +1878,10 @@ static int acquire_resource_from_hw_enabled_state(
 
 	// tg_inst not found
 	if (i == pool->stream_enc_count)
-		return false;
+		return -1;
 
 	if (tg_inst >= pool->timing_generator_count)
-		return false;
+		return -1;
 
 	if (!res_ctx->pipe_ctx[tg_inst].stream) {
 		struct pipe_ctx *pipe_ctx = &res_ctx->pipe_ctx[tg_inst];