drm/i915: Add a small wrapper to check for CCS modifiers.

Code looks cleaner with modifiers hidden inside this wrapper.
v2: Remove const qualifier (Ville)

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180822193827.6341-1-dhinakaran.pandiyan@intel.com