drm/i915/gt: Add some debug tracing for context pinning

Add the context pin/unpin events to the trace for post-mortem debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625194859.28005-1-chris@chris-wilson.co.uk