OMAPDSS: use DISPC register to detect context loss

Instead of relying on the OMAP specific
omap_pm_get_dev_context_loss_count() to detect register context loss, we
can achieve the same in a much simpler way by just observing the DISPC
registers.

We always set DISPC's load mode to LOAD_FRAME_ONLY, which is not the
reset value. Thus we can just observe the load mode to see if we have
lost register context.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>