drm/i915/selftest: Create mock_destroy_device

Just some prep work before we rework the lifetime handling, which
requires replacing all the drm_dev_put in selftests by something else.

v2: Don't go with a static inline, upsets the header tests and
separation.

Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918132505.2316382-2-daniel.vetter@ffwll.ch