msm-fb: display: lm2 writeback support on mpq platfroms

lm2 writeback has broken on mpq platfroms as lm1 and lm2 are
selecting the same axi writeback path. It adds the support
to select the right axi writeback path based the availability
of the write master

Change-Id: Ia6b868ff13de5edc641e4795794286893c063e66
Signed-off-by: Nagamalleswararao Ganji <nganji@codeaurora.org>