drm/amd: Fix spelling mistake "throtting" -> "throttling"

There is a spelling mistake in variable throtting_events, rename
it to throttling_events.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>