drm/amd: Mark some tables as const

This patch marks some compile-time constant tables 'const'.
The tables marked in this patch are the low hanging fruit
where little other changes were necesary to avoid casting
away constness etc. Also mark some tables that are private
to a file as static.

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>