drm/i915/blt: don't assume pinned intel_context

Currently we just pass in bcs0->engine_context so it matters not, but in
the future we may want to pass in something that is not a
kernel_context, so try to be a bit more generic.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
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>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190810091748.10972-1-chris@chris-wilson.co.uk