drm/atmel-hlcdc: check stride values in the first plane

The statement always evaluates to true since the struct fields
are arrays. This has shown up as a warning when compiling with
clang:
  warning: address of array 'desc->layout.xstride' will always
      evaluate to 'true' [-Wpointer-bool-conversion]

Check for values in the first plane instead.

Fixes: 1a396789

 ("drm: add Atmel HLCDC Display Controller support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180617084826.31885-1-stefan@agner.ch