msm_fb: display: Do a pre-fill to writeback buffers.

When switching mdp operating mode to blt, writeback buffers need
pre-fill. Move the pre-fill code from isr routine to do_blt, where
actual switch to blt mode happens.

CRs-fixed: 471098
Change-Id: I0f845397894c749ac518a1fa605f84cd5fc2ad15
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>