arm64: perf: Clean up enable/disable calls

Reading this code bordered on painful, what with all the repetition and
pointless return values. More fundamentally, dribbling the hardware
enables and disables in one bit at a time incurs needless system
register overhead for chained events and on reset. We already use
bitmask values for the KVM hooks, so consolidate all the register
accesses to match, and make a reasonable saving in both source and
object code.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>