drm/i915: Perform manual conversions for plane uapi/hw split, v2.

get_crtc_from_states() is called before plane_state is copied to uapi,
so use the uapi state there.

intel_legacy_cursor_update() could probably get away with looking at
the hw state, but for clarity always look at the uapi state.

Changes since v1:
- Convert entirety of intel_legacy_cursor_update (Ville).

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191031112610.27608-8-maarten.lankhorst@linux.intel.com