msm: mdss: fix DSI PHY timing configuration logic

In the current implementation, when MDSS driver checks
if there is a change in display resolution, then it sets
PHY timings update variable to true. But after this it
is never set to false once PHY timings have been updated.
This might cause issue since the variable always remains
true even if there is no change in display resolution.
Fix this issue by setting PHY update variable to false,
as soon as PHY timings has been updated.

Change-Id: Iab2cb8dbabdb53b7bba612f3062919803ecbd698
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>