drm/i915: refactor skylake scaler code into new file.

This moves the code from various places and consolidates it
into one new file.

v2:
- rename skl_program_plane -> skl_program_plane_scaler (Ville)
- also move skl_pfit_enable, and consequently make some skl_scaler_*
  functions static to skl_scaler.c (Ville)

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2fa703ffc7b96a41c392fd5ebbd2e6e4ffb6fb05.1612536383.git.jani.nikula@intel.com