msm_fb: display: prefill two writeback frames for lcdc.

Pre-fill two writeback frames before enable writeback mode
to avoid flickering for lcdc.

CRs-fixed: 325327
Change-Id: I2ae6edea4578441f2027cc7657c851cf9646a28c
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>