msm: mdss: Fix the ctx num for virtual ctl

When virtual ctl is allocated during simultaneous wb line and block mode
operation, the virtual ctl num should be the same as original ctl. This
is essential for color space conversion programing.

Change-Id: I1294a8f813e5b534cf10f9d4f721b42ce0bb6141
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>