drm/msm: 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 <tzimmermann@suse.de>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>