drm/i915: Do not clear mappings beyond VMA size

Do not to clear mappings outside the allocated VMA under any
circumstances. Only clear the smaller of VMA or object page count.

This is required to allow creating partial object VMAs which in
turn are needed for partial GGTT views.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>