drm: xlnx: zynqmp_disp: layer may be null while releasing

layer->info can be null if we have an error on the first layer in
zynqmp_disp_create_layers

Fixes: 1836fd5e ("drm: xlnx: zynqmp_dpsub: Minimize usage of global flag")
Signed-off-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241028133941.54264-1-lists@steffen.cc