hwc/overlay: Get rot dest dimensions instead of manipulating source

Currently for Mdss rotator we manipulate the layer buffer dimensions
to be equal to what rotator's destination dimensions would be, so that
these could be fed to MDP. Instead, this patch introduces APIs in
rotator so that hwc can directly query destination crop, width, height,
format from rotator.

This also simplifies the updateSource() and configRotator() helpers.

Change-Id: I501d998f2e0574683c764af9422742b2426ba0c7