drm/tinydrm: Replace tinydrm_of_find_backlight with of_find_backlight

Remove tinydrm_of_find_backlight from tinydrm-helpers.c. We now have
a generic of_find_backlight defined in backlight.c. Let the callers
of tinydrm_of_find_backlight call of_find_backlight. Also, remove
select BACKLIGHT_LCD_SUPPORT and select BACKLIGHT_CLASS_DEVICE from
tinydrm/Kconfig as it is a hack that is no longer needed.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/11dd1cabd098a730d07ab04c5987b139d14d8b21.1516810725.git.meghana.madhyastha@gmail.com