msm: mdss: fix PPP hang with RGB888 buffer format

Set the source buffer format to RGBX for solid fill case
for MDP3 to avoid PPP hang. Since the source data is not
fetched for solid fill this change will not have any
other issues.

Change-Id: I31f1e233ba8f307a22ac04f37c7804c8e4dc95b1
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>