ebb: Fix paf enable/disable branches

This patch fixes the paf_ebb_[enable/disable]_branches functions by
avoid them to set/reset the Global Enable (GE) bit. For enable case
if GE bit is set along with PME bit it will trigger a EBB branch.
ebb: Reset MMCR0 independently of PAF_EBB_FLAGS_RESET_PMU

This patch makes __paf_ebb_ebb_hook clear MMCR0[PMAO] and set set
MMCR0[PMAE] indenpedently if PAF_EBB_FLAGS_RESET_PMU is used.
ebb: test_ebb_pmu_clobber code cleanup
ebb: Add some comments to reset_mmcr0