drm: Rename drm_mode_object_get()

Subsequent patches will introduce reference counting APIs that are more
consistent with similar APIs throughout the Linux kernel. These APIs use
the _get() and _put() suffixes and will collide with this existing
function.

Rename the function to drm_mode_object_add() which is a slightly more
accurate description of what it does. Also the kerneldoc for this
function gives an indication that it's badly named because it doesn't
actually acquire a reference to anything.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170228144643.5668-2-thierry.reding@gmail.com