drm/i915: Replace drm_gem_object_{un/reference} with {put,get} functions

This patch unifies the naming of DRM functions for reference counting
of struct drm_gem_object. The resulting code is more aligned with the
rest of the Linux kernel interfaces.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618110154.30462-3-tdz@users.sourceforge.net