msm_fb: display: set max clk level when more than 2 pipes used

Correct logic to take all pipes into account and increase clk perf level
to max when more than 2 pipes are used.

Change-Id: Ibe6594e23a95e72c113b98c8f08b33395677d212
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>