drm/i915: Split cursor check_plane into i845 and i9xx variants

The 845/865 and 830/855/9xx+ style cursor don't have that
much in common with each other, so let's just split the
.check_plane() hook into two variants as well.

v2: Keep the common stuff in one place (Chris)
v3: s/DRM_FORMAT_MOD_NONE/DRM_FORMAT_MOD_LINEAR/

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-9-ville.syrjala@linux.intel.com