OMAP: DSS2: DSI: Represent L4 and VP as sources of VC instead of modes

The enum type dsi_vc_mode is a bit misleading as L4 slave port and video port
are sources to VC rather than the mode of operation. Rename then enum type and
its members. Merge dsi_vc_config_vp() and dsi_vc_config_l4() into a single
function called dsi_vc_config_source() which takes dsi_vc_source enum as an
extra argument.

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