msm_fb: display: fix the underrun issue when playing interlaced videos

Interlaced videos require higher mdp clk rate and the current code
sets mdp clk to max and return. However mdp clk calculation may still
give a result higher than max mdp clk, for this case the result is
kept and used later to enable blt mode to avoid underrun.

Change-Id: I185d0d15e5b79e94561e790e41713cdb405f8bf4
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>