disp: msm: sde: correct the available mixers count provided

On connecting Display port when primary is in suspend state, RM
provides all the available mixers for DP mode validations.
Mixers which are primary and secondary display configured are
also provided for DP mode, which can cause commit failures on
external DP. Add changes to provide the correct mixer count
in such cases.

Change-Id: I239feb60a5373545c0ab929a0c318c6c2ac66531
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>