drm/i915: Fix plane allocation/free functions

Use intel_plane_destroy_state in intel_plane_free to free the state.
Also fix intel_plane_alloc() to use __drm_atomic_helper_plane_reset(),
to get sane defaults from the atomic core.

This is needed to get the correct alpha value and blend mode from the
core, and any new default values added from new properties.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: b2081525 ("drm/i915: Add plane alpha blending support, v2.")
[mlankhorst: Update commit description to mention alpha blend support]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181113092804.13304-1-maarten.lankhorst@linux.intel.com