drm/gma500: Fix mem leak of cursor objects on Poulsbo

All dumb allocated buffers are now automatically pinned so no extra
pinning is needed. Also properly unreference gem objects so they can be
freed up properly. This only affects Poulsbo chips.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
drm/gma500: Fix mem leak of cursor object on Cedarview

All dumb allocated buffers are now automatically pinned so no extra
pinning is needed. Also properly unreference gem objects so they can be
freed up properly. This only affects Cedarview chips.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>