drm/i915: ensure the force pipe A quirk is actually followed

Many BIOSen forget to turn on the pipe A after resume (because they
actually don't turn on anything), so we have to do that ourselves when
sanitizing the hw state.

I've discovered this due to the recent addition of a pipe WARN that
takes the force quirk into account.

v2: Actually try to enable the pipe with a proper configuration instead
of simpyl switching it on with whatever random state the bios left it
in after resume.

v3: Fixup rebase conflict - the load_detect functions have lost their
encoder argument.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>