drm/i915: Switch intel_legacy_cursor_update() to intel_ types

Prefer the intel_ types in intel_legacy_cursor_update() over the
drm_ types. Should make it easier to adapt this to the uapi vs. hw
state split.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927131432.15978-1-ville.syrjala@linux.intel.com