OMAP: DSS2: Use PM runtime & HWMOD support

Use PM runtime and HWMOD support to handle enabling and disabling of DSS
modules.

Each DSS module will have get and put functions which can be used to
enable and disable that module. The functions use pm_runtime and hwmod
opt-clocks to enable the hardware.

Acked-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>