s390/cpum_sf: add diagnostic sampling event only if it is authorized

The SF_CYCLES_BASIC_DIAG is always registered even if it is turned of in the
current hardware configuration.  Because diagnostic-sampling is typically not
turned on in the hardware configuration, do not register this perf event by
default.  Enable it only if the diagnostic-sampling function is authorized.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>