msm_fb: display: flush base layer pipe when solid fill is enabled

Need to flush bgpipe with solid fill format to avoid fetching from
bg pipe when its contents are not displayed. Also add other missing
flushes.

CRs-Fixed: 342961
Change-Id: I03e517e02c02d9abc26fd9365e7b67025fa59657
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>