drm/i915: Restrict plane loops to only operate on overlay planes (v2)

Ensure that existing driver loops over all planes do not change behavior
when we begin adding new types of planes (primary and cursor) to the DRM
plane list in future patches.

v2: Switch to using drm_for_each_legacy_plane()

Cc: Intel Graphics Development <intel-gfx@lists.freedesktop.org>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>