drm/i915: Introduce intel_plane_state_reset()

For the sake of symmetry with the crtc stuff let's add
a helper to reset the plane state to sane default values.
For the moment this only gets caller from the plane init.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107142417.11107-5-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>