mdss: dsi: ensure proper clearing of DSI RDBK registers

During high performance scenarios, sometimes the DSI RDBK registers
are not getting cleared. This can cause improper read return values
since the RDBK data count will not get reset in such cases. Add
memory barriers during reset of RDBK registers to ensure that
the registers are cleared.

Change-Id: I870744b58c3e4064ca9f04f92e831d69139336db
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>