fbdev: msm: Avoid reserve of mixers for single layer mixer

Avoid reserving mixer for target having only one layer mixer.
Check for nunber of active layer mixer's present and avoid
allocating mixers to external or pluggable display if active
layer mixers are more than one.

Change-Id: I1ff7cff03808b65bb2e5ae6bdc897f48c6b1d5cd
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>