drm/i915: Get slice height before computing rc params

We need slice height to calculate few RC parameters
hence assign slice height first.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514153711.2359617-6-matthew.d.roper@intel.com