drm/amdgpu/pm_runtime: update usage count in fence handling

Increment the usage count in emit fence, and decrement in
process fence to make sure the GPU is always considered in
use while there are fences outstanding.  We always wait for
the engines to drain in runtime suspend, but in practice
that only covers short lived jobs for gfx.  This should
cover us for longer lived fences.

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