drm/i915: Fix cmdparser STORE/LOAD command descriptors

Fixes regression from
commit f1afe24f
Author: Arun Siluvery <arun.siluvery@linux.intel.com>
Date:   Tue Aug 4 16:22:20 2015 +0100

    drm/i915: Change SRM, LRM instructions to use correct length

which forgot to account for the length bias when declaring the fixed
length.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91844


Reported-by: Andreas Reis <andreas.reis@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Arun Siluvery <arun.siluvery@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>