drm/i915/gt: Replace opencoded i915_gem_object_pin_map()

As we have a pin_map interface, that knows how to flush the data to the
device, use it. The only downside is that we keep the kmap around, as
once acquired we keep the mapping cached until the object's backing
store is released.

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/20200708173748.32734-2-chris@chris-wilson.co.uk