drm/i915: Pass the crtc to skl_compute_dbuf_slices()

skl_compute_dbuf_slices() has no use for the crtc state, so
just pass the crtc itself.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122205633.18492-3-ville.syrjala@linux.intel.com