drm/amdgpu: Fix MMIO access page fault

Add more guards to MMIO access post device
unbind/unplug

Bug: https://bugs.archlinux.org/task/72092?project=1&order=dateopened&sort=desc&pagenum=1
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>