msm: mdss: Limit assertive display BL updates to certain modes

Assertive display requires updates when backlight changes only in mode
MDSS_AD_MODE_AUTO_STR. When in AUTO_BL mode, the backlight is modulated by
the assertive display block and should not be notified when the backlight
changes.

Change-Id: I6f37b10836047df3ed9bcd801fd10fc549084595
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>