msm: mdss: Correctly calculate DSI clocks if fbc is enabled

In cases where frame-buffer compression(fbc) is enabled, the
compressed ratio for xres isn't considered. Add logic to correctly
account for fbc when needed.

Change-Id: I0eec90cbbe0a0ffaea94cf1c2d6231e6545403b6
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>