msm: mdss: clear RDBK_DATA registers before every read command

Currently, we clear the RDBK_DATA registers after we perform
every read. This is to clear the read count in the register so
that the count starts from zero for the next read. But, the read
count can increment for non-read use cases as well such as
non-read BTA, error conditions of non-read commands. Hence, clear
the RDBK_DATA registers before every read.

Change-Id: I1197451f5ca42e9235a81d256f075c2c3b8cc05c
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>