msm_fb: display: Flush background pipe after solid fill configuration

For is_fg support on a foreground pipe, the  background pipe is
currently set to Solid Fill mode without being flushed. Add flush
after enabling Solid Fill on background pipe.

CRs-fixed: 352855
Change-Id: I91c4e391c9577a9198d048d2b369369d348efeab
Signed-off-by: Amara Venkata Mastan Manoj Kumar <manojavm@codeaurora.org>