drm/i915: remove intel_crtc_cursor_set_obj() (v5)

Merge it into the plane update_plane() callback and make other
users use the update_plane() functions instead.

The fb != crtc->cursor->fb was already inside intel_crtc_cursor_set_obj()
so we fold intel_crtc_cursor_set_obj() inside intel_commit_cursor_plane()
and merge both paths into one.

v5 (by Matt):
 - Rebase onto latest di-nightly codebase
 - Drop extra unreference call when we fail to pin (Ville)

Reviewed-by(v4): Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>