drm/i915: Sanitize stolen memory size calculation

On GEN8+ (not counting CHV) the calculation can in theory result in an
incorrect sign extension with all upper bits set. In practice this is
unlikely to happen since it would require 4GB of stolen memory set
aside. For consistency still prevent the sign extension explicitly
everywhere.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-6-git-send-email-imre.deak@intel.com