video: tegra-dc: add panel_set_backlight call

Tegra DC driver does not call panel_set_backlight, which can
result in absence of backlight on device. Fix this by calling
panel_set_backlight with BACKLIGHT_DEFAULT just after
panel_enable_backlight.

Tested-by: Robert Eckelmann <longnoserob@gmail.com> # ASUS TF101 T20
Tested-by: Nicolas Chauvet <kwizart@gmail.com> # Paz00
Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS TF T30
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # LG P895 T30
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>