drivers/perf: arm_pmu_acpi: drop redundant acpi_disabled check

acpi_disabled has been checked in armv8_pmu_driver_init and it shall
be ZERO in arm_pmu_acpi_probe, clean up this unnecessary check.

Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>