OMAPDSS: fix dss_get_ctx_loss_count for DT

When using DT, dss device does not have platform data. However,
dss_get_ctx_loss_count() uses dss device's platform data to find the
get_ctx_loss_count function pointer.

To fix this, dss_get_ctx_loss_count() needs to be changed to get the
platform data from the omapdss device, which is a "virtual" device and
always has platform data.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>