drm/i915: WARN_ON fence pin leaks

The fence pin count should always be <= the bo pin count. If that's
not the case then we have a funny problem and are leaking references
somewhere.

Which means we can catch fence pin leaks by checking for the same
upper limit as we do for the bo pin count. Inspired by a discussion
with Ville about a fence leak igt testcase.

v2: Also check for fence->pin_count <= ggtt_vma->pin_count, since that
might catch a leak even quicker. Also de-inline them, they're getting
too big.

v3: Don't separately check for MAX_PIN_COUNT since the > vma->pin_count
check will catch that already (Chris).

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