drm/i915/gtt: Allocate va range only if vma is not bound

When we have bound vma into an address space, the layout
of page table structures is immutable. So we can be absolutely
certain that if vma is already bound, there is no need to
(re)allocate a virtual address range for it.

v2: - add sanity checks and remove superfluous GLOBAL_BIND set
    - we might do update for an unbound vma (Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90224


Testcase: igt/gem_exec_big #bdw
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>