drm/i915: fixup overlay checks for interlaced modes

The drm core _really_ likes to frob around with the crtc timings and
put halfed vertical timings (in fields) in there. Which confuses the
overlay code, resulting in it's refusal to display anything at the
lower half of an interlaced pipe.

Tested-by: Christopher Egert <cme3000@gmail.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>