x86/resctrl: Remove hard code of RDT_RESOURCE_L3 in monitor.c

Scope of monitoring may be scoped at L3 cache granularity (legacy) or
at the node level (systems with Sub NUMA Cluster enabled).

Save the struct rdt_resource pointer that was used to initialize
the monitor sections of code and use that value instead of the
hard-coded RDT_RESOURCE_L3.

No functional change.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Peter Newman <peternewman@google.com>