msm: mdss: add WB support to use 2 LM for 4k usecase

Fix writeback to use 2 LM for resolution width more than 2048.
And update the pinfo and var data structures when the writeback
output resolution is changed.

CRs-fixed: 930504
Change-Id: Id096511a26845a962c30c6ebf2102ffcb0c18a5f
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>