msm: mdss: Do not divide horizontal porches when fbc is enabled

MDP was dividing the porch values for Frame-Buffer Compression(FBC) by
the compression ratio incorrectly. Only the x-res needs to be divided.
This fixes the mismatch of timing values between DSI and MDP.

Change-Id: I8c57d807ac9d2b451031641da7971b4c33a66a36
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>