x86/psr: fix bug which may cause crash

x86/psr: fix bug which may cause crash

During test, we found a crash on Xen with below trace.
(XEN) Xen call trace:
(XEN)    [<ffff82d0802a065a>] R psr.c#l3_cdp_write_msr+0x1e/0x22
(XEN)    [<ffff82d0802a0858>] F psr.c#do_write_psr_msrs+0x6d/0x109
(XEN)    [<ffff82d08023e000>] F smp_call_function_interrupt+0x5a/0xac
(XEN)    [<ffff82d0802a2b89>] F call_function_interrupt+0x20/0x34
(XEN)    [<ffff82d080282c64>] F do_IRQ+0x175/0x6ae
(XEN)    [<ffff82d08038b8ba>] F common_interrupt+0x10a/0x120
(XEN)    [<ffff82d0802ec616>] F cpu_idle.c#acpi_idle_do_entry+0x9d/0xb1
(XEN)    [<ffff82d0802ecc01>] F cpu_idle.c#acpi_processor_idle+0x41d/0x626
(XEN)    [<ffff82d08027353b>] F domain.c#idle_loop+0xa5/0xa7
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 20:
(XEN) GENERAL PROTECTION FAULT
(XEN) [error_code=0000]
(XEN) ****************************************

The bug happens when CDP and MBA co-exist and MBA COS_MAX is bigger
than CDP COS_MAX. E.g. MBA has 8 COS registers but CDP only have 6.
When setting MBA throttling value for the 7th guest, the value array
would be:
    +------------------+------------------+--------------+
    | Data default val | Code default val | MBA throttle |
    +------------------+------------------+--------------+

Then, COS id 7 will be selected for writting the values. We should
avoid writting CDP data/code valules to COS id 7 MSR because it
exceeds the CDP COS_MAX.

Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>