tests/libkms-test-plane: fix possbile memory leak

The pointer p aquired with drmModeGetPlane() is not free in error
path. Fix possible memory leak by calling drmModeFreePlane() in
the error path.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>