drm/i915: remove ACPI related DRM_ERRORs

They're not really errors (well actually I don't know; I don't
understand _DSM and _MUX well enough to say, but I do know they spam
people's logs and seem to be harmless).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: The _DSM error got remove in another patch already]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44250
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>