drm/i915: unreference default context on module unload

Before module unload is called, gpu_idle() will switch
to default context. This will increment ref count of base
object as the default context is 'running' on module unload
time. Unreference the drm object so that when context
is freed, base object is freed as well.

v2: added comment to explain the refcounts (Ben Widawsky)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>