msm: mdss: fix initial mixer reserve logic for WB path

Commit a5f4c89e2774c070350039e3a9d9349908475a7c ("msm:
mdss: select mixer respective to ctl path for wb") tries
to reserve the initial mixer path for primary display
and use rest of the path for external and WB display.
It does not decrease the number of available mixers
after updating the mixer pool. This leads to invalid
memory access when all available mixers are in use.

Ideally, it should check the number of available dspp
which are reserved for primary display while allocating
the mixer for writeback path.

Change-Id: Ib97291d907dc8261005bca3cd947fd907f90d773
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>