msm: mdss: set the default mdp clcok rate to max supported by target

Set the default mdp clk rate to be used in mdp_probe at max supported.
For certain targets such as 8939 which support destination split and
use 2k panel the minimum clock rate required will be 320MHz. This will
apply only until first update comes from the userspace.

Change-Id: Ia4bf56401eaec86ffd8338b5dd0c8b730ad4b1fc
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>