drm: Reject getfb for multi-plane framebuffers

getfb can only return a single plane, so reject attempts to use it with
multi-plane framebuffers.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Daniel van Vugt <daniel.van.vugt@canonical.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 308e5bcb ("drm: add an fb creation ioctl that takes a pixel format v5")
Cc: stable@vger.kernel.org # v3.3+
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105518
Link: https://patchwork.freedesktop.org/patch/msgid/20180320225839.30905-1-daniels@collabora.com