msm: mdss: Fix hardware cursor configuration for right mixer

The right mixer for hwcursor was not configured, making it to work
only on dsi0. This change configures the left and right mixers
appropriately based on single/dual dsi.

Change-Id: Ib12589777b3b7a2a202447c89bb079ffb969cf7c
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>