drm/i915: grab a pages pin count for preallocate stolen

But only when we indeed set up a gtt mapping. We need this since the
vma also holds a pages_pin_count, on top of the unconditional
pages_pin_count we grab for all stolen objects (to avoid swap-out).

This should avoid a pages_pin_count underrun when cleaning up
framebuffers objects taken over from the BIOS.

Chris mentioned in his review that this bug even predates the vma
conversion.

Reported-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>