drm/vc4: Set ->is_yuv to false when num_planes == 1

drm/vc4: Set ->is_yuv to false when num_planes == 1

When vc4_plane_state is duplicated ->is_yuv is left assigned to its
previous value, and we never set it back to false when switching to
a non-YUV format.

Fix that by setting ->is_yuv to false in the 'num_planes == 1' branch
of the vc4_plane_setup_clipping_and_scaling() function.

Fixes: 670c7768d2828 ("drm/vc4: Add support for YUV planes.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181009132446.21960-1-boris.brezillon@bootlin.com