drmmode_display: remove old zpos ioctl fallback
drmmode_display: remove old zpos ioctl fallback

Now that we've backported the zpos property to 3.4, we no longer
need the old zpos ioctl fallback.

BUG=chromium:222980
TEST=Verified that cursor works on 3.8 and still works on 3.4.

Change-Id: I90de2e1b9c7d633496ad4e30f9066ce78a3a51f7
Signed-off-by: Mandeep Singh Baines <19acf4ef1f391f4aab404dc5703cac3632e06f3a@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/46958
Reviewed-by: John Sheu <e133b36fa071c0f7efb4cebd35fa1371d3ae789d@chromium.org>
Reviewed-by: Stéphane Marchesin <e16f69845633a0cf74affe1e5a677caa6296c937@chromium.org>

markyzq/armsoc-rockchip,markyzq/armsoc-rockchip