Pointer fb is being dereferenced when assigning dev before it
is null checked.  Fix this by only dereferencing dev after the
null check.

Fixes: 6b7ce2c4 ("drm/gma500: Remove struct psb_fbdev")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216162136.270114-1-colin.king@canonical.com