Perf: Reset pmu after hotplug

The counter enable bits were staying in undefined state after CPU
came out of hotplug when perf was not enabled.
This was resulting into non-zero values for counters in ftrace buffer
which were not enabled by perf.
Clear the pmu when CPU comes out of hotplug and perf is not enabled.

Change-Id: I31e17565ad914650a12102a5c359b363f8f2ca2a
Signed-off-by: Sheetal Sahasrabudhe <sheetals@codeaurora.org>