drm/i915: Move intel_atomic_state_free() into intel_atomic.c

Move intel_atomic_state_free() next to its counterpart.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-13-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>