crocus: Support offset query for multi-planar planes
Multi-planar planes can be imported from VA-API or V4L2. In this case we
would currently report wrong values for gbm_bo_get_offset().
This does not fix any know visible bug, as crocus hardware usually does not support muliti-planar planes in the display engine - in which case the wrong values would make Mutter glitch. Lets report correct values regardless.
While on it, also use the helper function for counting planes.
See also:
This has been tested on a ThinkPad T400 with P8400 / GM45 - the bug I was investigating was a different one, however, and has a MR pending for a couple of years already: https://github.com/intel/intel-vaapi-driver/pull/514