drm/i915: do not assert DE_PCH_EVENT_IVB enabled

The current code was checking if all bits of "val" were enabled and
DE_PCH_EVENT_IVB was disabled. The new code doesn't care about the
state of DE_PCH_EVENT_IVB: it just checks if everything else is 1.

The goal is that future patches may completely disable interrupts, and
the LCPLL-disabling code shouldn't care about the state of
DE_PCH_EVENT_IVB.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: I think the commit message is actually wrong in it's
description of what the old test checked, but the new one seems sane.
So meh.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>