drm/i915/dsb: Extract assert_dsb_has_room()

Pull the DSB command buffer size checks into a small helper so
we don't have repeat the same thing multiple times.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-6-ville.syrjala@linux.intel.com


Reviewed-by: Animesh Manna <animesh.manna@intel.com>