gma500: Fix SDVO turning off randomly

Some Poulsbo cards seem to incorrectly report SDVO_CMD_STATUS_TARGET_NOT_SPECIFIED instead of SDVO_CMD_STATUS_PENDING, which causes the display to be turned off.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>