drm/gma500: fix null dereference of pointer fb before null check

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