drm/amdgpu: Make amdgpu_ctx_mgr_entity_fini static

Function amdgpu_ctx_mgr_entity_fini() only has a single local caller so
lets make it local.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>