drm/amdgpu: Fix NULL ptr dereference issue for non userq fences

Add the correct fences count variable [num_fences] in the fences
array iteration to handle the userq / non-userq fences.

v2:(Christian)
  - All fences in the array either come from some reservation object
    or drm_syncobj. If any of those are NULL then there is a bug
    somewhere else.

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>