drm/i915: migrate skl planes code new file (v5)

Rework the plane init calls to do the gen test one level higher.

Rework some of the plane helpers so they can live in new file,
there is still some scope to clean up the plane/fb interactions
later.

v2: drop atomic code back, rename file to Ville suggestions,
add header file.
v3: move scaler bits back
v4: drop wrong new includes (Ville)
v5: integrate the ccs gen12 changes
v6: fix unrelated code movement (Ville)

Signed-off-by: Dave Airlie <airlied@redhat.com>
[Jani: fixed up sparse warnings.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4e88a5c6b9ab3b93cc2b6c7d78c26ae86f6abbd0.1612536383.git.jani.nikula@intel.com