vgaarb: Fix VGA decodes changes

When VGA decodes change we need to do a bit more evaluation of exactly what
has changed.  We don't necessarily give up all the old owns resources and
we need to account for resources with locks.  The new algorithm is: If
something is added, update decodes.  If legacy resources were added and
none were there before, we have a new participant.  If something is
removed, update decodes.  If we previously owned it, we no longer own it.
If it was previously locked, invalidate all locks and release it.  If
legacy resources were removed and none are left, remove the participant
from VGA arbitration.

Previously we updated decodes, released ownership of everything that was
previously decoded, ignored all locks, and went off looking for another
device to transfer VGA to.  In a test case where Intel IGD removes only
legacy VGA memory decoding, this left the arbiter switching to discrete
graphics without actually disabling legacy VGA IO from the IGD.  As a
bonus, we bumped up the count of VGA arbitration participants for no
good reason.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Kill now unused variables, reported by the 0-day kernel
builtbot.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>