drm/i915: Don't drop pinned fences

drm/i915: Don't drop pinned fences

Userspace can currently provoke this when e.g. trying to use a pinned
scanout as a cursor or overlay target. Later on that might lead to
some fun fence pin count mayhem.

Spurred by Ville's report that something goes wrong here and
originally I've thought that this might slip through the pwrite gtt
fastpath. But that one checks of obj tiling, so should be ok.

But one thing that _does_ blow up is the vma unbinding with more than
one address space. The next patch will fix this.

v2: Use a WARN_ON - Chris pointed out that we already catch all cases
so userspace can't provoke this like I've originally feared.

While reviewing relevant code I've noticed a pile of DRM_ERROR in the
overlay&cursor code which are all triggerable by userspace. Tune them
down while at it.

v3: Split out the DRM_ERROR->DRM_DEBUG_KMS change into a separate patch,
as requested by Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>