msm: mdss: add HW recommended delay for dfps feature.

When prefetch is enabled, MDSS HW works on 2 vsync
boundaries i.e. mdp_vsync and panel_vsync. In the current
implementation we are only waiting for mdp_vsync and there
might be scenarios where driver is configuring interface
flush before panel_vsync, which might lead to undefined
behavior in MDSS HW. So add the recommended delays before
and after dfps update to avoid such scenarios.

Change-Id: Idc801f7b63ea85d88949c8ded06fae322f90dbdb
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>