msm: mdss: fix wb_ctl_alloc to allow use of rotator2

Currently MDSS HW can support maximum of 5 active ctl paths, so the size
of the container array holding all ctl paths is also 5. Now current code
has a limitation where it doesn't allow rotator2 to be used if number of
DSPP based interfaces are 4, which is the case for apq8084. This can lead
to sub-par performance when more than 1 rotator sessions are needed
simultaneously. Fix this by allowing last 2 spots of the ctl path
container array for rotator.

Change-Id: I40e968141d92ce6272072d9b5b8c59402d8a543c
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>