drm/msm: support backoff time update only in sde rsc

SDE RSC driver updates the complete RSC configuration when
client requests for only timer updates like static wrapper
or pdc/rsc backoff time. This may reset unintentional settings
on SDE rsc while client calls it during dynamic resolution
switch or inline rotation. This patch adds a new API to update
only timer configuration on sde rsc with new client settings.

Change-Id: I15c0e75cc4008431dc622dcf29085e9839dee15d
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>