msm: mdss: fix issue in calculating decimated dimensions

Width and height are not properly calculated when decimation
is involved. When there is scaling on a pipe, the required number
of pixels are wrongly programmed resulting in pipe hangs and
leading to TLB SYNC issues. This change handles proper
calculation of decimated width and height.

Change-Id: Iadc4b201b32dda3dba6afe23070f349677be417c
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>