drm/amd: make some function-local tables static const

These tables were initialized on stack on each call, avoid that
and save a little bit of text size.

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>