drm/i915: Move programming plane scaler to its own function.

This cleans the code up slightly, and will make other changes easier.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-8-maarten.lankhorst@linux.intel.com