sna/gen2+: Remove assertion on draw->type for fills

sna/gen2+: Remove assertion on draw->type for fills

I overlooked the tiling-fill path passing down a temporary DrawableRec
into the fill_boxes callback - invalidating the assertion.

Fixes regression from
commit 43176b9bfafe389c4b9ed676f4e50e3b06f858c4
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Jun 30 21:01:11 2014 +0100

    sna/dri2: Pass around the correct DrawableRec for sampling from the foriegn bo

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70461#c62
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>