msm: mdss: refactor command mode vsync control logic

Current logic is prone to corner case when mdss_mdp_remove_vsync_handler
is called twice: once from mdss_mdp_cmd_stop and again from vsync ctrl
logic. Need to properly identify the second call and avoid resetting the
rdptr ticks which prevent from clocks being turned off properly.

Change-Id: I9eeebd0c16e67a249508980427c81fa2e339edec
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>