loader: fix the no-modifiers case

Normally modifiers take precendence over use flags, as they are more
explicit.  But if the driver supports modifiers, but the xserver does
not, then we should fallback to the old mechanism of allocating a buffer
using 'use' flags.

Fixes: 069fdd5f
Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
(cherry picked from commit c56fe411)