drm/i915: Fix a NULL vs IS_ERR() bug

The mmap_offset_attach() function returns error pointers, it doesn't
return NULL.

Fixes: eaee1c08

 ("drm/i915: Add a function to mmap framebuffer obj")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ZH7tHLRZ9oBjedjN@moroto