drm/tinydrm: Convert tinydrm_enable/disable_backlight to backlight_enable/disable

Remove tinydrm_enable/disable_backlight and let the callers call the
more generic backlight_enable/disable helpers

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/ec700724e47814e6426966e093bd32b2364ba147.1516810725.git.meghana.madhyastha@gmail.com