drm/i915: Fail if we can't get a fence for gen2/3 tiled scanout

Gen2/3 display engine depends on the fence for tiled scanout. So if we
fail to get a fence fail the entire operation.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-2-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>