drm/i915/gen9+: Fix initial readout for Y tiled framebuffers

If BIOS configured a Y tiled FB we failed to set up the backing object
tiling accordingly, leading to a lack of GT fence installed and a
garbled console.

The problem was bisected to
commit 011f22eb

 ("drm/i915: Do NOT skip the first 4k of stolen memory for pre-allocated buffers v2")
but it just revealed a pre-existing issue.

Kudos to Ville who suspected a missing fence looking at the corruption
on the screen.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: <ronald@innovation.ch>
Cc: <stable@vger.kernel.org>
Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reported-by:  <ronald@innovation.ch>
Tested-by:  <ronald@innovation.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108264
Fixes: bc8d7dff

 ("drm/i915/skl: Provide a Skylake version of get_plane_config()")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016160011.28347-1-imre.deak@intel.com