drm/amdgpu: individualize fence allocation per entity

Allocate fences for each entity and remove ctx->fences reference as
fences should be bound to amdgpu_ctx_entity instead amdgpu_ctx.

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