drm/i915: Convert BUGs to WARNs in the video overlay code

BUG is bad, just use WARN.

Also drop one BUG(!overlay) since we'd oops anyway when dereferencing
it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>