drm/i915/ringbuffer: Make context pin/unpin symmetric

Currently, we have a special routine for pinning the context state at
the start of activity tracking, but lack the complementary unpin
routine. Create it to to ease later patches that want to do partial
teardown on error, and, not least, to improve the readability of the
code.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605085348.3018-1-chris@chris-wilson.co.uk