drm/i915: Extract intel_crtc_dbuf_weights()

Extract the code to calculate the weights used to chunk up the dbuf
between pipes. There's still extra stuff in there that shouldn't be
there and must be moved out, but that requires a bit more state to
be tracked in the dbuf state.

v2: Keep crtc_state->wm.skl.ddb

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/20210122205633.18492-7-ville.syrjala@linux.intel.com