cpupower: Condense pstate enabled bit checks in decode_pstates()

The enabled bit (bit 63) is common for all families so we can remove
the multiple enabled checks based on family and have a common check
for HW pstate enabled.

Signed-off-by: Nathan Fontenot <nathan.fontenot@amd.com>
Reviewed-by: Robert Richter <rrichter@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>