drm/radeon: Select BACKLIGHT_LCD_SUPPORT

Explicitly select BACKLIGHT_LCD_SUPPORT to satisfy the direct dependency
of BACKLIGHT_CLASS_DEVICE.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>