msm: mdss: cleanup mdss command interface code

Remove unused or unnecessary variables and make sure each variable
that tracks rdptr, mdp_clk, pingpong or vsync status has a single
purpose. Slow fps, suspend/resume failure, unbalanced clk cnt and no
vsync event issues get fixed, and fps deviation is improved.

CRs-fixed: 486116
Change-Id: Id33945c0ce6cde368bd58df7e8755d7e9ff6add7
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>