drm/i915: Remove unnecessary check for unsupported modifiers for NV12

There is already a check to allow only RGB8888 formats with CCS
modifiers.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628061854.6430-1-dhinakaran.pandiyan@intel.com