drm/amd/powerplay: Mark pem_event_action chains as const

As these arrays were of pointer to pointer type, they were
pointer to pointer to const. Make them pointer to const
pointer to const.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nils Wallménius <nils.wallmenius@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>