drm/msm/sde: Add api to get number of dsc encoders in topology

Expose an api to query on number of dsc encoders required to support
a given topology in SDE driver.

Change-Id: I63aac35f2044f5251053e697ac796caad33372a7
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>