msm: mdss: Avoid splash buffer allocation from generic heap for MSM8909w

MDP3 driver was allocation the splash buffer from generic CMA heap,
resulting in modem boot failure. Removed mdp3_alloc call and use generic
mdss_alloc implementation to handle splash allocation.

Change-Id: I6850173d29c659c9152d38beac507a79706d1a9d
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>