msm_fb: display: consolidate overlay_commit and display_commit

This patch consolidate overlay_commit into display_commit to
support iommu buffer be used as frame buffer

Change-Id: Idc33a340b7cf199c8e2f5e7f09b901d1502fac9e
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>