drm: xlnx: zynqmp_dpsub: Enable plane in atomic update

Unconditionally enable the DPSUB layer in the corresponding atomic plane
update callback. Setting the new display mode may require disabling and
re-enabling the CRTC. This effectively resets DPSUB to the default state
with all layers disabled. The original implementation of the plane atomic
update enables the corresponding DPSUB layer only if the framebuffer
format has changed. This would leave the layer disabled after switching to
a different display mode with the same framebuffer format.

Signed-off-by: Anatoliy Klymenko <anatoliy.klymenko@amd.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240523-dp-layer-enable-v2-1-d799020098fc@amd.com