drm: Remove pending_read_domains and pending_write_domain

The last user of these (i915.ko) no longer does. We can slim down the
core GEM object by removing the unused 8 bytes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170705154900.28697-1-chris@chris-wilson.co.uk