powerpc/perf: Avoid FAB_*_MATCH checks for power9

Since power9 does not support FAB_*_MATCH bits in MMCR1,
avoid these checks for power9. For this, patch factor out
code in isa207_get_constraint() to retain these checks
only for power8.

Patch also updates the comment in power9-pmu raw event
encode layout to remove FAB_*_MATCH.

Finally for power9, patch adds additional check for
threshold events when adding the thresh mask and value in
isa207_get_constraint().

fixes: 7ffd948f ('powerpc/perf: factor out power8 pmu functions')
fixes: 18201b20 ('powerpc/perf: power9 raw event format encoding')
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>