arm: perf: krait: stop using singleton PMU

Currently the krait_pmu_{enable,disable}_event functions use the global
cpu_pmu variable while all the other pmu enable/disable functions
derive this from the event argument.

This patch brings the Krait functions into line with the rest of the PMU
backends by deriving the address of the pmu from the event argument.

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Tested-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>