drm/i915: Introduce skl_plane_wm_level accessor.

For future Gen12 SAGV implementation we need to
seemlessly alter wm levels calculated, depending
on whether we are allowed to enable SAGV or not.

So this accessor will give additional flexibility
to do that.

Currently this accessor is still simply working
as "pass-through" function. This will be changed
in next coming patches from this series.

v2: - plane_id -> plane->id(Ville Syrjälä)
    - Moved wm_level var to have more local scope
      (Ville Syrjälä)
    - Renamed yuv to color_plane(Ville Syrjälä) in
      skl_plane_wm_level

v3: - plane->id -> plane_id(this time for real, Ville Syrjälä)
    - Changed colorplane id type from boolean to int as index
      (Ville Syrjälä)
    - Moved crtc_state param so that it is first now
      (Ville Syrjälä)
    - Moved wm_level declaration to tigher scope in
      skl_write_plane_wm(Ville Syrjälä)

v4: - Started to use enum values for color plane
    - Do sizeof for a type what we are memset'ing
    - Zero out wm_uv as well(Ville Syrjälä)

v5: - Fixed rebase conflict caused by COLOR_PLANE_*
      enum removal

v6: - Do not use skl_plane_wm_level accessor in skl_allocate_pipe_ddb

v7: - Get rid of wm_uv, which is not used in skl_plane_write_wm(Ville)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513093816.11466-2-stanislav.lisovskiy@intel.com