msm: mdss: fix BWC stride calculations

BWC stride should account for both chroma components for RAU size and
subsequently the stride for the compressed stream. At the same time
since the luma and chroma pixels are stored in same plane, update
calculations to properly get plane size.

CRs-Fixed: 493063
Change-Id: Ied618e9b4c1d82868a018531c8b2e3d4ea1df16d
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>