drm/amdgpu: simplify eviction fence suspend/resume

The basic idea in this redesign is to add an eviction fence only in UQ
resume path. When userqueue is not present, keep ev_fence as NULL

Main changes are:
 - do not create the eviction fence during evf_mgr_init, keeping
   evf_mgr->ev_fence=NULL until UQ get active.
 - do not replace the ev_fence in evf_resume path, but replace it only in
   uq_resume path, so remove all the unnecessary code from ev_fence_resume.
 - add a new helper function (amdgpu_userqueue_ensure_ev_fence) which
   will do the following:
   - flush any pending uq_resume work, so that it could create an
     eviction_fence
   - if there is no pending uq_resume_work, add a uq_resume work and
     wait for it to execute so that we always have a valid ev_fence
 - call this helper function from two places, to ensure we have a valid
   ev_fence:
   - when a new uq is created
   - when a new uq completion fence is created

v2: Worked on review comments by Christian.
v3: Addressed few more review comments by Christian.
v4: Move mutex lock outside of the amdgpu_userqueue_suspend()
    function (Christian).
v5: squash in build fix (Alex)

Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>