OMAPDSS: clean up the omapdss platform data mess

The omapdss pdata handling is a mess. This is more evident when trying
to use device tree for DSS, as we don't have platform data anymore in
that case. This patch cleans the pdata handling by:

- Remove struct omap_display_platform_data. It was used just as a
  wrapper for struct omap_dss_board_info.
- Pass the platform data only to omapdss device. The drivers for omap
  dss hwmods do not need the platform data. This should also work better
  for DT, as we can create omapdss device programmatically in generic omap
  boot code, and thus we can pass the pdata to it.
- Create dss functions for get_ctx_loss_count and dsi_enable/disable_pads
  that the dss hwmod drivers can call.

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