drm/amdgpu: Remove duplicated "context still alive" check

When amdgpu_ctx_mgr_fini() calls amdgpu_ctx_mgr_entity_fini() it contains
the exact same "context still alive" check as it will do next. Remove the
duplicated copy.

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>