msm: mdss: maintain backlight-related variables in mfd structure

Backlight status are framebuffer specific and should be maintained in
mfd data structure. Otherwise, the release of one fb (e.g. fb1)
prevents backlight updates of another fb (e.g. fb0).

CRs-fixed: 520695
Change-Id: I7baaebda22a6ae0c7c391d5898c5a7e2467be0be
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>