OMAP: DSS2: DISPC: Fix context save/restore

The current method of saving and restoring the context could cause a
restore before saving, effectively "restoring" zero values to registers.

Add ctx_valid field to indicate if the saved context is valid and can be
restored.

Also restructure the code to save the ctx_loss_count in save_context(),
which makes more sense than the previous method of storing new
ctx_loss_count in dispc_need_ctx_restore.

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