drm/i915: Clean up the sprite modifier checks

Split the g4x and snb cases into separate functions to match how we deal
with all other platforms. Also sort the switch cases to match the format
lists we've declared earlier, to ease comparisons.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171222192231.17981-5-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>