drm/i915/cx0: Add intel_cx0_get_owned_lane_mask()

There are more parts of C10/C20 programming that need to take owned
lanes into account. Define the function intel_cx0_get_owned_lane_mask()
and use it. There will be new users of that function in upcoming
changes.

BSpec: 64539
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230814131331.69516-2-gustavo.sousa@intel.com