lib: sbi_pmu: Before using we should ensure PMU init done

If trap earlier before sbi_pmu_init done, some path would call
sbi_pmu_ctr_incr_fw, then it would go wrong:
1. if phs_ptr_offset is zero, then it get a wrong pmu state ptr
2. if phs_ptr_offset is ok, but we didn't call pmu_set_hart_state_ptr
it would be NULL POINT

Of course, the above situation will not occur at present, but it is
reasonable to check before using.

Signed-off-by: yang.zhang <yang.zhang@hexintek.com>
Reviewed-by: Anup Patel <anup@brainfault.org>