drm/i915: Clean up some cdclk switch statements

Redo some switch statements in the cdclk code to use a common
fall through for the default case. Makes everything look a bit
more uniform

Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024095216.1638-2-ville.syrjala@linux.intel.com