make amdgpu_activate() return early when called during hibernate resume

make amdgpu_activate() return early when called during hibernate resume

Hibernate does DVACT_QUIESCE/DVACT_SUSPEND from hibernate_resume(),
before config_process_deferred_mountroot() calls amdgpu_attachhook().

Without this change the amdgpu_device_prepare() call ends up faulting in
ttm_resource_manager_evict_all() after the
'drm/amd: Evict resources during PM ops prepare() callback'
commit from April.

earlier diff from and ok stsp@
report and bisection from florian@