msm_fb: display: alloc/free writeback buffer done at do_blt()

To avoid double freeing writeback at end of blt mode, let
both alloc and free writeback buffer done at do_blt().

Change-Id: Ida5d24589c19be8f6f655eca6add00badca356b2
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>