OMAP: DSS2: DSI: Introduce generic write functions

Intoduce enum "dss_dsi_content_type" to differentiate between DCS and generic
content types.

Introduce short and long packet write functions which use generic
Processor-to-Peripheral transaction types. These are needed by some devices
which may not support corresponding DCS commands. Create common write functions
which allow code reuse between DCS and generic write functions.

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