OMAP: DSS2: rewrite use of context_loss_count

The function to get device's context loss count has changed from

omap_pm_get_last_off_on_transaction_id() to
omap_pm_get_dev_context_loss_count()

Change name of the function pointer in omapdss.h accordingly, and use
the term "context loss count" instead of "context id" in the code.

Restructure the context loss count functions to handle errors properly,
and ensure that context is always considered lost if an error happens.

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