drm/amd/powerplay: mark phm_master_table_* structs as const

Also adjust phm_construct_table to take a const pointer

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>