OMAP: DSS2: DSI: Introduce sync_vc functions

The DSI protocol engine has no interrupt for signalling the end of a Frame
transfer. The present approach is to send a BTA after DISPC generates a
FRAMEDONE interrupt, and unlock the dsi bus only when the BTA Ack is received.

The assumption made with this approach was that OMAP will send a BTA only after
the long packet corresponding to the last line is sent. However, it is possible
that on the DISPC FRAMEDONE interrupt there are 2 (or more) lines of pixel data
in the DSI line buffer. Hence, the BTA Ack could be received for the long packet
corresponding to the second last line (or the third last and so on..).
Therefore, the current method doesn't ensure that the complete frame data is
sent before we start a new transfer. A similar explanation holds valid if we
send a BTA in between multiple short/long command packets from the slave port.

Introduce dsi_sync_vc functions, based on Tomi Valkeinen's idea, which ensure
that the DSI Virtual Channel in use(update_channel) completes its previous work
before proceeding to the next Frame/Command.

For a frame update, the DSI driver now sends a callback to the Panel Driver
on the FRAMEDONE interrupt itself. The callback in the panel driver then unlocks
the bus. dsi_sync_vc() functions are placed in dsi_vc_config_l4() and
dsi_vc_config_vp() to ensure that the previous task of the Virtual Channel is
completed.

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

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