msm: mdss: mdp: mixer allocation for pluggable primary display

Try to allocate mixers LM0 and LM1 to an external pluggable
display if it is configured as primary display.

Change-Id: I81672132a6452535dcd7c2977e76f88a5f3aecf9
Signed-off-by: Narender Ankam <nankam@codeaurora.org>