cpupower: Fix coredump on VMWare

cpupower crashes on VMWare guests.  The guests have the AMD PStateDef MSR
(0xC0010064 + state number) set to zero.  As a result fid and did are zero
and the crash occurs because of a divide by zero (cof = fid/did).  This
can be prevented by checking the enable bit in the PStateDef MSR before
calculating cof.  By doing this the value of pstate[i] remains zero and
the value can be tested before displaying the active Pstates.

Check the enable bit in the PstateDef register for all supported families
and only print out enabled Pstates.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Stafford Horne <shorne@gmail.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>