i915: Fix SDVO potentially 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.

This could also happen to i915.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>