drm/i915: avoid unused scale_user_to_hw() warning

drm/i915: avoid unused scale_user_to_hw() warning

After the function is no longer marked 'inline', there
is now a new warning pointing out that the only caller
is inside of an #ifdef:

drivers/gpu/drm/i915/display/intel_panel.c:493:12: warning: 'scale_user_to_hw' defined but not used [-Wunused-function]
  493 | static u32 scale_user_to_hw(struct intel_connector *connector,
      |            ^~~~~~~~~~~~~~~~

Move the function itself into that #ifdef as well.

Fixes: 81b55ef1f47b ("drm/i915: drop a bunch of superfluous inlines")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428213106.3139170-1-arnd@arndb.de