drm/i915/gtt: Check va range against vm size

Check the allocation area against the known end
of address space instead of against fixed value.

v2: Return ENODEV on internal bugs (Chris)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>