drm/i915: make structure intel_sprite_plane_funcs static

The structure intel_sprite_plane_funcs is local to the source
and does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'intel_sprite_plane_funcs' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811134938.4183-1-colin.king@canonical.com