msm: mdss: fix mdp vbif qos priorities

Current driver read if any nrt vbif is present
and if it is not, the number of qos priority
levels gets wiped out, causing the driver
not to program the vbif qos priority levels for
the rt client. Fix this problem by making sure
that if a vbif nrt does not exist in the platform,
driver only ignore the nrt settings, but keeps
programming the vbif rt.

Change-Id: I2fe5eea5a2ff852b22a59cdb0139b17f3dd7c494
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>