OMAPDSS: Taal: remove external backlight support

Taal panel driver supports two kinds of backlight control: 1) using DSI
commands sent to the panel to control the backlight, 2) calling function
pointers going to the board file to control the backlight.

The second option is a bit hacky, and will no longer be needed when the
PWM driver supports the backlight features. After that we can use the
standard PWM backlight driver.

This patch removes the second backlight control mechanism, and adds a
boolean field, use_dsi_backlight, to nokia_dsi_panel_data which the
board file can use to inform whether the panel driver should use DSI
commands to control the backlight.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>