msm: mdss: update PHY timing calculation logic for resolution change

In current implementation we are always overriding the PHY read
from dt entry with the ones we are calculating in driver. Update
the PHY timing only when there is a change in resolution.

Change-Id: I2ba936af6e55b27c7b28d38990b32e896c877e08
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>