mam_fb: display: add per stage alpha drop detection

Currently, alpha drop detection is done per frame which
means multiple blending stages share same alpha drop status
whcih does not reflect alpah effect truly on each blending
stage. This patch enforce alpha drop detection is doen per
blenging stage so that alpha effect applied properly at
each blending stage.

Change-Id: I90f8f6395c88e665b64ae90e64b65cea1d54b41a
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>