msm: mdss: Send backlight sysfs notification in all BL update locations

There are two places where the backlight can get updated. Currently, the
sysfs notification only happens in mdss_fb_set_backlight. If the backlight
is restored during the first display commit, the backlight update occurs
in mdss_fb_update_backlight. This change adds the sysfs notification in
update_backlight.

Change-Id: Iffa1e4f0aa0cdc8babb1b6683d3175795f4b4d27
Signed-off-by: Benet Clark <benetc@codeaurora.org>
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>
CRs-fixed: 949146