drm/i915: Mask reserved bits in display/sprite address registers

The purpose of this patch is to avoid zeroing the lower 12 reserved bits
of surface base address registers (framebuffer & sprite).  There are bits
in that range that may occasionally be set by BIOS or by other components.

Signed-off-by: Armin Reese <armin.c.reese@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>