drm/i915: Pass plane to watermark calculation functions

Sometimes we might need to change the way we calculate
watermarks, based on which particular plane it is calculated
for. Thus it would be convenient to pass plane struct to those
functions.

v2: Pass plane instead of plane_id
v3: Do not pass plane to skl_cursor_allocation(Ville Syrjälä)
v4: - Make intel_crtc_get_plane static again(Ville Syrjälä)
    - s/cursor_plane/plane(Ville Syrjälä)
    - Pass plane to skl_compute_wm_* instead of plane_id(Ville Syrjälä)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124090653.14547-2-stanislav.lisovskiy@intel.com