msm: mdss: Use TPG to handle null commit

Add support to use TPG to send configurable constant color to
panel during the null commit to ensure that no fetch
is happening during the secure to non-secure transition and
vice-versa.

Change-Id: I8cc1a283e88fa9be91da38019c7d532d5180dfb5
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>