drm/amdgpu: fix xgmi perfmon a-b-a problem

Mapping hw counters per event config will cause ABA problems so map per
event instead.

v2: Discontinue starting perf counters if add fails.  Make it clear what's
happening with pmc_start.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Harish Kasiviswanathan <harish.kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>