drm/i915: Move skl plane fb related checks into a better place

Move the skl+ specific framebuffer related checks from
intel_plane_atomic_check_with_state() into a new function
(skl_plane_check_fb()) which we'll simply call from the skl
plane->check() hook.

v2: Split out the Y/Yf+CCS vs. interlaced change (José)

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907152413.15761-11-ville.syrjala@linux.intel.com