OMAPDSS: Use only "omapdss_dss" platform device to get context lost count

OMAPDSS: Use only "omapdss_dss" platform device to get context lost count

When enabling a hwmod, omap_hwmod refers to the register mentioned in the
hwmod struct's member 'prcm.omap4.context_offs' to see whether context was
lost or not. It increments the context lost count for the hwmod and then clears
the register.

All the DSS hwmods have the same register(RM_DSS_DSS_CONTEXT) as context_offs.
When DSS is enabled, the first hwmod to be enabled is the "dss_core" hwmod since
it's corresponding platform device is the parent platform device("omapdss_dss").
The dss_core hwmod updates it's context lost count correctly and clears the
register. When the hwmods corresponding to the children platform devices are
enabled, they see that the register is clear, and don't increment their context
lost count. Therefore, all the children platform devices never report a loss in
context.

The DISPC driver currently gets the context lost count for DSS power domain from
it's corresponding platform device instance("omapdss_dispc"). The DISPC platform
device is one of the child devices, and it's corresponding hwmod("dss_dispc")
doesn't report the context lost count correctly.

Modify dss_get_ctx_loss_count() such that it always takes the "omapdss_dss"
platform device as it's input, move the function to dss.c so that it has access
to that platform device.

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