drm/amdgpu: Mark all instances of struct drm_info_list as const

All these are compile time constand and the
drm_debugfs_create/remove_files functions take a const
pointer argument.

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>