perf/arm_cspmu: Simplify initialisation

It's far simpler for implementations to literally override whichever
default ops they want to, by initialising to the default ops first. This
saves all the bother of checking what the impl_init_ops call has or
hasn't touched. Make the same clear distinction for the PMIIDR override
as well, in case we gain more sources for overriding that in future.

Reviewed-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/dd39718ee4890fd46a8e443c25303e87ae23f422.1706718007.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>