drm/i915: Extract framebufer CCS offset checks into a function

intel_fill_fb_info() has grown quite large and wrapping the offset checks
into a separate function makes the loop a bit easier to follow.

v2: Skip the check for non-CCS planes. (Mika)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191221120543.22816-4-imre.deak@intel.com