drm/i915: Extract intel_crtc_ddb_weight()

skl_ddb_get_pipe_allocation_limits() doesn't care how the weights
for distributing the ddb are caclculated for each pipe. Put that
calculation into a separate function so that such mundane details
are hidden from view.

v2: s/adjusted_mode/pipe_mode/

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/20210122205633.18492-2-ville.syrjala@linux.intel.com