drm/i915: s/assert_spin_locked/lockdep_assert_held/

assert_spin_locked() becomes an unconditionally compiled BUG_ON(),
adding debug code right into the heart of critical routines like
interrupt handlers.

   text	   data	    bss	    dec	    hex
1296480	  19944	   2272	1318696	 141f28	before (lockdep disabled)
1295984	  19944	   2272	1318200	 141d38	after

1336261	  21139	   3208	1360608	 14c2e0	before (lockdep enabled)
1339920	  21139	   3208	1364267	 14d12b	after

Small saving for release; hopefully more instructive in debug.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302132801.599-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>