drm/amdgpu: fix the indexing issue during rlcg access ctrl init

In case that the GET_INST() is used for looping, only loops for the
times of actual num of xcc, otherwise GET_INST() will return the invalid
index, a.k.a -1

And also remove the redundant mask checking in case of GET_INST()

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>