drm/i915: Add the missing Y/Yf modifiers for SKL+ sprites

drm/i915: Add the missing Y/Yf modifiers for SKL+ sprites

Y/Yf were dropped out from the SKL+ sprite modifier list on account
of some watermark issues Daniel Stone was having. My subsequent testing
seemed to indicate that things work better now, so add the modifiers
back in.

v2: Update the commit message with a better explanation

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/20171222192231.17981-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>