OMAP: DSS2: Functions to request/release DSI VCs

Introduce functions which request and release VC's. This will be used in panel
drivers in their probes.

omap_dsi_request_vc() takes in the pointer to the omap_dss_device, the VC_ID
parameter which goes into the header of the DSI packets, and returns a Virtual
channel number (or virtual channel register set) which it can use.

omap_dsi_set_vc_id() takes the omap_dss_device pointer, the Virtual Channel
number and the VC_ID that needs to be set for the specifed Virtual Channel.

omap_dsi_release_vc() takes the omap_dss_device pointer and the Virtual Channel
number that needs to be made free.

Initialisation of VC parameters is done in dsi_init().

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