OMAP: DSS2: DSI: Split dsi_vc_dcs_read() into 2 functions

Split dsi_vc_dcs_read() into the functions:
- dsi_vc_dcs_send_read_request(): This is responsible for sending the short
packet command with the read request.
- dsi_vc_dcs_read_rx_fifo(): This parses the DSI RX fifo of the given virtual
channel, identifies the type of data received, and fills a buffer with the data
provided by the panel.

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