When trying to work with NV12 buffers, I'm seeing lots of the
following noise:

10-20 20:39:40.855   474   474 E hwc-drm-two: Failed to import layer, ret=-22
10-20 20:39:40.888   474   474 E hwc-platform-drm-generic: could not create drm fb -22
10-20 20:39:40.888   474   474 E hwc-platform-drm-generic: Failed to rm fb
10-20 20:39:40.888   474   474 E hwc-drm-utils: Failed to import buffer

Digging back it seems we're setting three fds for 2 plane
formats.

This patch changes it so we only set two, which stops the
import errors.

(NOTE: I'm still not seeing the NV12 buffers rendered properly,
 but I'm  not yet sure where that issue is stemming from).

Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: Id5493e37f17053740ca6b643fd838a2d11507462