drm: msm: sde: Allow reservation of odd number of DSCs

Add a check to escape the DSC allocation once the requested
number of DSCs has been reached and a pair is not required
for the last DSC allocated. This issue was introduced when
trying to allow for quad DSC, which broke single DSC use.

Change-Id: I4bc368004f92570d588e76ceb832d63fd3bb15d7
Signed-off-by: Thomas Dedinsky <tdedinsk@codeaurora.org>