fbdev: msm: Reserve mixers with dspp for primary display

Avoid allocating mixers LM0 and LM1 to external display and
writeback as they are allocated to primary display first.

Change-Id: I343a70dca2e664e5c9765fa2f0b933714b747b4a
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>