renderonly: fix dumb BO allocation for non 32bpp formats

Take into account the resource format, instead of applying a hardcoded
32bpp. This not only over-allocates 16bpp formats, but also results in
a wrong stride being filled into the handle.

Fixes: 848b49b2 ("gallium: add renderonly library")
CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 0c71a19f)