video: mx3fb: always enable BACKLIGHT_LCD_SUPPORT

Commit 7edaa761

 ("video: mx3fb: Add backlight control support")
changed the mx3fb driver so it always selects the BACKLIGHT_CLASS_DEVICE
symbol, but that is hidden behind BACKLIGHT_LCD_SUPPORT in Kconfig, so
we get a Kconfig warning for multi_v5_defconfig, which doesn't have that:

Warning: (DRM_RADEON && DRM_NOUVEAU && DRM_I915 && DRM_GMA500 &&
DRM_SHMOBILE && DRM_TILCDC && FB_BACKLIGHT && FB_MX3 && USB_APPLEDISPLAY
&& FB_OLPC_DCON && ASUS_LAPTOP && SONY_LAPTOP && THINKPAD_ACPI &&
EEEPC_LAPTOP && ACPI_CMPC && SAMSUNG_Q10) selects BACKLIGHT_CLASS_DEVICE
which has unmet direct dependencies (HAS_IOMEM && BACKLIGHT_LCD_SUPPORT)

This makes sure we always enable both symbols together for mx3fb, like
we do for the other drivers that can't be built without backlight
support. Note that a better solution would be to ensure the driver can
work with or without backlight support.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>