drm/i915/gtt: Avoid calling non-existent allocate_va_range

drm/i915/gtt: Avoid calling non-existent allocate_va_range

On hsw and older, we do not need to allocate the ppgtt on the fly and so
ppgtt->allocate_va_range() is NULL. Fixup ppgtt_bind_vma not to call it,
in that case!

v2: PIN_UPDATE still exists.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180601093554.13083-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180601093554.13083-2-chris@chris-wilson.co.uk