msm: mdss: remove play_cnt reset in overlay set

play_cnt is used to manage currently displayed buffer, this value is
incremental and should be initialized only once for new pipe creation,
but overlay set can be called after pipe creation to change params.
Remove reset of play_cnt to avoid reset on subsequent overlay set calls.

Change-Id: Idef90a6123c4e1a9e8b1ab963bf78330cdd912bf
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>