msm: mdss: replace kmalloc with kzalloc wherever appropriate

Currently different drivers within MDSS, allocate dynamic memory using
kmalloc and then use explicit memset to zero out the content. Instead use
kzalloc API to achieve the same.

Change-Id: I09d3d5e5986206f90c97d4d04fc3a855c233a7ae
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>