OMAPDSS: DSI: flush posted write in send_bta

Flush posted write after setting the bit to send the BTA to ensure the
BTA is sent right away, as the code in dsi_vc_send_bta_sync() waits for
an interrupt caused indirectly by sending the BTA.

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