drm/i915: don't leak the pin_map on error

Add some onion to populate_lr_context.

v2: prefer err_unpin_ctx
    drop the fixes tag, worst case we just spew a warn before everything
    is cleaned up and balance is restored

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180301114639.510-1-matthew.auld@intel.com