drm/i915/display: Update small joiner ram size

Xelpd supports larger small joiner ram.

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805101937.14664-1-vandita.kulkarni@intel.com