msm: mdss: observe dual pipe mux inefficiencies for mdp clock rate

There is some inefficiency in dual pipe mux where it can only use 8 out
of 9 clocks when running in dual pipe mode. In order to account for
this, need to ensure that minimum MDP clock accounts for this
inefficiency.

Change-Id: Ia7bebaaff7720cd293cd478f7f45ca8ed8d36f2f
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>